In a galaxy not so far far away....

The Andromeda galaxy is presided over by two major powers - the Y'Ing and the Y'Ang. These powers have existed together but never integrated with each other into a galactic whole. They have been in a Cold War state for millennia. In spite of this, they have roughly the equivalent state of technology and share of the galaxy and share the galaxy roughly equally:

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For the past 5 years, the Y'Angs have been mobilising an invasion force with the aim of taking the whole galaxy over, once and for all.

Facts (at least in this fiction):

  • The Andromeda galaxy has an average radius of 220,000 light years
  • Faster Than Light travel exists
  • Faster Than Light communications also exists, and transmits twice as fast as travel does
  • It takes six months for a message to travel from the core to the rim
  • Travel and comms get progressively more difficult as you reach the core (one or more supermassive black holes has something to do with this)
  • The Y'Ings and the Y'Angs each have about a habitable billion star systems and a billion Super Star Destroyers and a wide range of supporting fleets
  • FTL in this fiction does break physics/time. Think of Star Wars/Star Trek where FTL is simply getting somewhere else really, really quickly
  • Comms cannot be transmitted or received while a ship is in FTL travel
  • Defending forces have sensors that can detect enemy FTL travel (but not comms) within 10 light-years of the sensor

Y'Ing/Y'Ang political personalities - Analogous to America/Russia during the Cold War. Y'Ang is basically Russia.


How do the Y'Ings defend against the Y'Ang invasion? What command structure would be in place, and what would a high level defence strategy look like?

  • $\begingroup$ Are the 6 months comm travel from core to rim a perfect time, or are they a real time (under influence of the mentioned super heavy stuff)? $\endgroup$
    – dot_Sp0T
    Commented Mar 9, 2017 at 11:55
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    $\begingroup$ This FTL detection, does it detect FTL transit within that 10Ly radius or merely exits from FTL travel within that radius? $\endgroup$
    – Green
    Commented Mar 9, 2017 at 12:33
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    $\begingroup$ Defensive strategy will depend upon the culture and values of the defender. Some cultures will happily use an aggressive spoiling defense, others psyops and deterrence, and still others super-strong planetary defenses (knowing that some will undoubtedly be overwhelmed). Communication lag means that fleet admirals and system marshals will have a high degree of autonomy to run their war as they see fit. Together, they suggest a vast war of attrition likely to fundamentally alter both empires. Political factions among the invaders might just topple their leadership to prevent that. $\endgroup$
    – user535733
    Commented Mar 9, 2017 at 13:20
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    $\begingroup$ @Pᴇᴛᴇ ...which at 25 light years per hour, that gives you 24 minutes of warning before a fleet arrives. 10 ly / (25 ly/h) = 24 minutes. $\endgroup$
    – Yakk
    Commented Mar 9, 2017 at 15:47
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    $\begingroup$ FTL comms at 2 x travel speed would make things very interesting. It's almost back to wars in Napoleonic times in terms of the relationship between the speed of communication and of travel. You'd certainly need a highly devolved command structure; battles across several light-years taking place dozens or hundreds of light-years from HQ would be over before word got back that they'd started. $\endgroup$ Commented Mar 9, 2017 at 16:44

6 Answers 6


How to defend against the Spanish (Y'Ang) Armada

1. Have spies find out when the Y'Ang Armada is setting sail.

This would make the situation a lot easier. In the case of the Spanish Armada, the fact that the Pope granted the Spanish soldiers indulgences and allowed King Phillip of Spain to collect a crusade tax was kind of a dead giveaway. The logistical requirements for collecting a billion super star destroyers in one spot are pretty incomprehensible, so I think you could get some advance warning.

Speaking of which...have you looked at how much a super star destroyer weighs? These nerds say ~10e13 kg. Times a billion star destroyers is ~10e22 kg which is basically the mass of Luna. You could potentially upset the orbits of a system with a fleet that big. Better spread them out a bit.

2. Get advance warning of the fleet's appearance

From Wikipedia:

The fleet was not sighted in England until 19 July, when it appeared off The Lizard in Cornwall. The news was conveyed to London by a system of beacons that had been constructed all the way along the south coast.

Beacons do 'signal' at the speed of light, but lighting one beacon then another in succession isn't really the fastest. So your FLT comms plus some long range sensors should do the trick. The English fleet was stationed about 90 miles away from The Lizard in Plymouth, so it was on hand to react quickly. The bottom line is, while the Galaxy is a big place, due to the nature of FTL travel there may only be a few places the enemy could reasonably show up. Same with the Spanish Armada; Britain has a lot of coastline, but there were only a few directions the Armada could reasonably approach from.

3. Concentrate your forces

In order to threaten an invading fleet of a billion, you need to preserve your core fleet in one group. While the nature of your naval combat is uncertain, the effect of superior numbers has had a multiplicative effect for all naval battles since the invention of gunnery. That is, since with gunnery you can engage at range, any fleet at a 2:1 (or greater) disadvantage will quickly be pounded into space dust since the enemy has so many more weapons. This contrast with, say, the movie 300 where there were a series of one on one (or rhino/giant/monster on one) engagements at the front line. If the 300 fought with guns, they would have died in a hail of lead 15 minutes into the movie.

The English managed to gather many of their best and brightest admirals together, such famous figures as Francis Drake, Charles Howard, and John Hawkins. By getting all the far-flung fleets and privateers concentrated the English fleet was able to put up a fight. And this far flung fleet could only be gotten through the use of the spies/pope to give advance notice of the intention to invade.

4. Hope for the best

Anyone who plays grand strategy games knows that once the battle starts, you just hope for the best. You can set up the fight at a location and time that advantage you, but when the metal meets the meat, anything can happen.

5. Attrition is your best friend

If you did win, great! The Spanish Armada was beaten, not broken. But then it was forced to retreat up and around Scotland (as the English fleet was blocking the Channel passage). This was not good, since it was now late September and storms were brewing off the Hebrides and the coast of Ireland. Nearly half the fleet got wrecked on the home voyage. The obvious analogy here is that if you win, try to cut off the enemy's easy escape path. Since the fleet is concentrated, this doesn't require much FTL capability; the victorious Admiral just needs to keep hounding the defeated enemy fleet until calamity befalls them.

If you lost, its ok! Its not over yet. That billion ship fleet still needs food and supplies (what happens to morale when the toilet paper runs out?) for untold trillions or quadrillions of crewmembers. If they do succeed in breaching your defenses and wrecking havoc in your core territories, you can harass their supply lines, play scorched Earth, and make planetary sieges as difficult as possible. Think of the Japanese Kamikazes, the Romans not giving in after Hannibal destroyed army after army, and the Russians burning Moscow before Napoleon arrives. This too does not require massive coordination, small forces acting independently are more dangerous as guerrilla forces. The defeated Admiral needs to get what remains of his fleet to safety so it can be a threat in the future (like Washington retreating to Valley Forge).

  • $\begingroup$ The question asks about Command Structure also. $\endgroup$ Commented Mar 9, 2017 at 16:50
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    $\begingroup$ @RBarryYoung Then there is plenty of meat left on the bone for other answerers to chew on. $\endgroup$
    – kingledion
    Commented Mar 9, 2017 at 16:54
  • $\begingroup$ Once again, logistics beat firepower... $\endgroup$
    – Mermaker
    Commented Mar 9, 2017 at 20:07
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    $\begingroup$ Or look at the british empire, age of sail. Timespan sounds familuar - it just had a little less communication than that. $\endgroup$
    – TomTom
    Commented Mar 10, 2017 at 14:16

There is really only 1 problem: communication speed. This means that due to long lag times you cannot do a central command system. Here is how I would defend:

Step One: Balance, use your intelligence agencies (preferably three letters) to find out where the enemy forces are. Use that to move your forces around to counter.

Step two: Draw them in, Unless you want to do preemptive strike, you will be reacting in the first stages. So go defence in depth (in 3D, maybe 4D). Draw them in, cut them off, defeat them in detail.

Step three: Let lose those soldiers that have been training for so long. Give the local commanders the space they need. Don't go president Johnson (or a long list of political leaders that tried to wage war from on high)

Step four: Jus in bello Treat the soldiers you capture well, that will make, barring some strange values, more willing to stop fighting. Sun Tzu has some ideas on that.

Step five: Fiddly bits; how to do the above with the tech you described.

  • $\begingroup$ I don't see step four. Those soldiers will also stop fighting if you sell them into slavery. I have heard that it is better to be feared than loved, in particular with respect to your enemies. $\endgroup$
    – kingledion
    Commented Mar 9, 2017 at 15:31
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    $\begingroup$ We have quite a bit of data on how humans fight and what makes it easier to surrender. Think of the Japanese during WW2 as a no surrender version. Or the opposite: Germans who surrendered more willingly to the Allies then the Communists, for some good reasons I say. *Aliens are alien, so who knows in this case. $\endgroup$ Commented Mar 9, 2017 at 15:35

Defensive strategy

The Y'Ing High Command will want to induce the Y'Ang to not attack. After all, you can't lose the fights that you don't engage in and the best wars are the ones where you don't have to fight. That said, the price of this war will be trillions of deaths and the destruction of entire star systems. There are four options for how this Cold War will end: It will continue as it is, it will detente without bloodshed, it will escalate and burn the galaxy, or the two empires decide that it's easier to just merge.

Y'Ing Objectives

  1. Preserve political autonomy
  2. Preserve population
  3. Preserve industrial capacity

Y'Ang Objectives

  1. Remove political autonomy from the Y'Ing.
  2. Preserve population
  3. Preserve industrial capacity

This could turn into a Dune-style War of Assassins where there need be no bloodshed of the common people but a lot of the empire leadership dies. Given the extreme costs of open conflict, empire leadership may decide that this is the best course. For the lives of the quadrillions of citizens of each empire, I hope they choose this path.

Defensive Screening

Establish screening pickets along the border and significantly above and below the galactic plane (don't want the Y'Ang to cheat and just go off-plane to avoid detection) There's enough population and industrial capacity to completely saturate all space with redundant detection posts. Extend these listening posts into Y'Ang territory as well. Replace listening posts as the Y'Ang detect and destroy them.

Saturate the Y'Ang Political and Military Organization with Spies/Informants

Knowing when a Y'Ang fleet departs from dry dock is a significant advantage when planning counter-movements. Any kind of positional information on enemy fleet or troop dispositions will help immensely.

Develop a "Nuclear" Triad

Develop the equivalent of nuclear submarines, bombers and ICBMs (heh, they'd be IGBMs, Inter-Galactic Ballistic Missiles). The threat of overwhelming force should make the Y'Ang think twice before launching an attack.

IGBMs - Position autonomous "nuclear" warheads near strategic targets. With the industrial capacity of the Y'Ing, it's possible to build thousands or IGBMs for each target.

Bombers - Position significant attack resources off the galactic plane with orders that if war broke out, they are to attack their targets.

Submarines - highly capable but small fleets of attack ships, each ship capable of glassing a planet, that constantly patrol through Y'Ang space without making threatening moves against any particular Space is huge so catching these patrols will be incredibly difficult.

Command and Control Structures

Trying to manage a naval fleet containing one billion Super Star Destroyers plus billions of support craft can't be done by one human. This many SSDs is only two orders of magnitude away from the number of cells in the human brain. Including support craft, the number of ships in the navy is the number of brain cells in the human brain.

The only solution to this is to automate everything about fleet command and logistics. I can't imagine the compute or storage resources required to do this but it's going to make the combined might of AWS, Google Compute Platform and Microsoft Azure seem like pet projects that junior engineers do as warmup projects.

But, luckily, the Y'Ing and the Y'Ang have had centuries to advance their distributed computing prowess and Strong AI. It's entirely possible that there will be no human commanders at all, just AIs.

Research Objectives

Pour as much money as possible into FTL Interdiction fields. Mobility is the key to winning most any conflict. If you deny your enemy the ability to move, you make them that much easier to find and to kill. Without interdiction fields, there's nothing to stop an IGBM from popping out of FTL right on top of your industrial bases and political centers with zero warning.

Make it in the best interests of the Y'Ang businesses not to attack the Y'Ing

If your business depends heavily on trade with the opposing empire, you'll be highly motivated to pressure your politicians to avoid armed conflict. The greater the financial and business relationships between the two empires the greater the resistance to any conflict that will disrupt those relationships.

As the Y'Ang chief executive will find, (s)he won't stay in power long if many major trade unions withdraw their support or start to actively undermine your administration.


Two billion known systems. All systems are known to both sides. All major planetary industrial centers known. Significant mobilized orbital industry.

Ring shaped battlefield, no one goes through the center.

Each battleship has a crew of ~50,000. Assuming that support staff is 8x the ship crews (assumes higher because of the greater complexity of space fleets.) This means that there are 50 trillion sailors in the Y'Ing navy on just the Super Star Destroyers. The naval support staff is 400 trillion people. Earth's infantry requires between 5x and 6x people for each infantryman.

USA military is about 1.8 million compared to 318 million. 0.5% of population is in the military. Estimated population of the Y'Ing empire is 1e+16 people or 1 quadrillion people. 1 quadrillion people.

Imperfect information game state, no one knows everything about all the pieces.

Mixed Industrial Mobility - Most planetside industrial capacity will be static. Reasonable to assume that orbital industrial facilities can be moved, given that there are a billion 1.6Km long battleships on each side.

First, this is unthinkably huge industrial capacity. On Earth in WW2, the US made approximately 47 billion bullets. That's only 47 times more things than the number of kilometer long battleships this civilization has made. Their bullet making capacity is in the quadrillions of bullets, easily.


The other answers cover strategy at length, so I'll answer command structure.

Obviously, you have a HQ. Every bit of information eventually gets back to them, and they are in charge of the general direction of the war and coordinating all the fronts. However, the further away they are from the front lines, the more out of touch they will be.

Given the nature of the galaxy, I'd foresee the war bubbling out in both directions around the core (assuming the core is untraversable like in Star Wars or other like galactic structures). So you'd have two Fleet Admirals. One for each direction. However, they would be able to override HQ's orders since they are more in touch with whats going on in their galactic direction.

Then, considering the time radius of the galaxy, you'd have maybe 5-6 Vice Admirals each with about 1 month/segment of space-time. They would be more in touch with their region than the Fleet Admiral, so they would be able to override their orders.

Finally, you'd have a couple Rear Admirals in each Galactic Segment who would report to their Vice Admiral, but ultimately be in-charge of their subsection.

Under them, the command structure would be as it usually is.

The idea is to have a command local enough for the time distortion to not be an issue. But you also need someone coordinating the entire war, otherwise it's just two dozen different groups fighting another two dozen different groups. This revised structure leads to an odd partially inverted hierarchy where one guy can ignore any order from lag-space. However, as soon as their commanding officer is within a certain distance (they are out of lag-space), the order needs to be followed.

Obviously, any order that tries to detail the means as well as the ends will fail at any rank above Sub-fleet Commander (think a squad leader in an army). So you need people directing the goals of any given Sub-fleet, and then the Sub-fleet reports up the chain so that everyone is in the loop.

  • $\begingroup$ I guess in addition to reporting up the chain, you should also report anything interesting directly to your same-level neighbors (who can base their decisions on this), because otherwise the enemy can travel faster than your information. $\endgroup$ Commented Mar 9, 2017 at 21:53
  • $\begingroup$ @PaŭloEbermann - Thats a good thought. Makes sense. Or at least has the immediate superiors send the information both up and down when they get it. $\endgroup$
    – amflare
    Commented Mar 9, 2017 at 22:18

The defence strategy:

Three lines for defense:

  1. Scattered recon
  2. Small fleets
  3. Big fleets

As the enemy approaches line #1. relays the information about the movements. #2 dodges the enemy and line #3 stops it. Line #2 joins the battle from the rear and behind. #1 and #2 have effectively cut the enemy off the supply.

This is only why the enemy cannot blitzkrieg. It will need to clear off at least #2 before it can penetrate further into the territory. To do this the enemy needs to scatter its troops. The idea how the scattering can be utilised is to locally create advantages of having more troops. Because the enemy would like to push, it will have to do some sacrifices to get anything done against line #1 that retreats all the time and the line #2 that has enough big fleets to eliminate too small scatterings.

Command structure wise the fleets have a simple missions they can carry out without micro-management. Thus they will carry out their missions automatically. Line #1 reports to line #2 and they report to line #3 that report to HQ.

This strategy eliminates too long FTL jumps, because you do not want to crash with missiles or whatever projectiles that are a real risk behind #1 line. Story-wise it can explain why there is a cold war and multiple offences without success.

This defense strategy corresponds closely to: panzer recon - reserves - arty structure. If the enemy is not much stronger they do not want to go too deep, because recon orders artillery bombardments and the reserves can block the enemy for the artillery to do more damage. Panzers with the recon make it so that they can escape and are scary against soft targets (like supply). I could easily see this becoming a stale mate if there would not be air forces. In your space situation there is not something similar to break the structure.


The primary issue with being on the defensive in a galactic scale war is that while you must be everywhere to defend, your enemy only needs to attack one or two places and can do so with overwhelming force.
The obvious solution to this would be a preemptive strike on the Y'Ang industrial worlds and repair and supply depots.
However, if this is not politically or militarily feasible, (There may be political opposition to starting the war and the targets I suggested are by definition surrounded by warships) your best option is to create a system defense mechanism similar to what the Havenites used in David Webers Honorverse series, with some additions made possible by the FTL comm system.
This will enable you to defend with a minimum of ships and simultaneously counterattack.

For system defense:

  1. Use a Moriarty unit (pre-fabbed sublight unit with nothing but missile control and the best stealth you can give it.) I suggest multiple Moriarties per system as they are toast as soon as they are detected.
    The purpose of the Moriarty is to control a TON of missile launched from pods. While pod launched missiles will have a much lower initial velocity then their ship launched counterparts, they have two major advantages over ships for system defense. First, with the Moriarty you will have the ability to launch missile volleys orders of magnitude larger than your enemies. Second, if you are using the pods in a system defense doctrine, the size constraints that limit them in ships are largely gone, enabling you to use more powerful drives and warheads. This means that while your missiles will have a much lower initial velocity (there are probably ways to overcome some of that) they will have a much higher final velocity and you'll be using more of them.
    Think water guns against a tsunami.
  2. FTL detection: As far as system defense goes, the FTL detectors will only give you 24 mins of warning that a potential enemy of unknown strength is heading in a given direction.
    While better than nothing, the lack of information will make it difficult to use as anything other than an early warning for the system that will be attacked.
    While you can deal with this by placing sensor platforms on areas that the Y'Ang are likely to use and extrapolate the targets from the data, the fact that it cannot detect the size of the enemy fleet means that it will be difficult to prioritize what systems to send your fleet to defend as the Y'Ang can simply send a drone equipped with an FTL drive across the galaxy setting off any detector in its path.
    My suggestion to deal with this would be either some sort of FTL intercept device (kind of hard to imagine that working without a lot of luck involved) orrrrr.. (Drumroll please) More Moriarty!!!! (I love missiles!!)
    Joking aside the real reason not to rely on this as anything other than a short-term warning system is that the only place the Y'Ang absolutely must end up in will be your systems. Any other assumption can be fatal. To plan based on what you think your enemy should do assumes that they have the same constraints that you do. (Enter Hannibal)
    This does not mean that it's not worth doing, only that it should not be relied upon, as unless you can cover half the galaxy with sensor units the Y'Ang can avoid them. Intelligence (spies) will work much better as a method of tracking enemy movements.

Command structure: @amflare did pretty well on this. All I want to add is that if the Y'Ang are as expansionist as you make them sound, there may be opportunities for your spies to start some "civil unrest" (read riots, mayhem, and possible revolts)
I'd like to paraphrase Sun Tzu on this one. Three steps

  1. Get someone competent
  2. Get them as much of what they need as you can
  3. Get out of the way

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