How would military look like in a world with only cities

Question

I am creating a world that consists only of cities. There is an 'outside' but the people never go there and the cities are connected with long underground tunnels.

There is a king, and he would of course need to have some kind of military power. But how would a military look like in this setting? There are no open battlefields to fight, or cavalry to win battles with. Siege weapons are also completely useless. So in what way would a military work? Will they have the same standard weapons, swords and shields, or will those be useless in the streets of a city and are there weapons better suited for this?

Edit: It is more a medieval setting than a modern one. There is one king, and a servant/representative of the king in each other city. The 'known world' is just one country, with a few big cities and a few smaller ones. The tunnels can fit a group of men, but not to much. In between the cities are smaller settlements and markets, in the tunnels underground. A trip via the tunnels between cities is approx. 2 weeks. No flight. The walls of the cities are also to big and high to be able to climb on, although there are inner walls pressed against the bigger ones for some kind of control.

Answer 1

Assuming no one can cheat or think outside the box because the outside is somehow toxic or hostile to human activities, then military forces will devolve into Infantry forces, and the Infantry forces will have their characters based on the wants and needs of the owning polity. (more on that later)

Since urban operations are pretty intensive, the first thing that will become clear to every force is the need for lots of manpower. Attackers will find themselves rapidly reduced to small, squad sized forces operating in very tenuous contact with the main body due to the nature of urban geography (once you turn the corner into an ally or enter a building, you are out of direct contact with the larger formation). Defenders have the same problem, in order to "cover" approaches and have forces available to meet the attackers, they will also need lots of manpower.

The next issue will be what sorts of weaponry will be appropriate? Fighting will be done at short ranges for the most part, so the bulk of the soldiers will have weapons that work best at close range. Since buildings act as impromptu fortifications, there will be a need to breach walls and clear closed spaces, so weapons like grenades, flamethrowers, short range rockets with large warheads and so on will be developed. In a medieval setting, many troops will become miners, sappers or engineers to do the same things with picks and shovels, making an opening for a grenadier to throw the flaming hand grenade into the hole...More modern settings will see the development of submachine guns, assault rifles and carbines, shotguns and handheld grenade launchers, or their futuristic equivalents (Metalstorm, anyone?).

Vehicles, if used, will be heavily armoured to withstand attacks at close range, including IED's, mines, suicide bombers, RPG's or whatever exists in your scenario. They will fight back with weapons that can deliver high volumes of fire, or explosive breaching charges. Engineer vehicles will also be heavily favoured to clear paths and demolish strongpoints. IDF vehicles like the ACHZARIT and PUMA have evolved in this direction for this reason. Tanks, if they exist, would probably evolve into smaller fire support vehicles much like German StuG's of the second world war, capable of moving down narrow streets and having a short barrelled weapon firing high explosive shells rather than a long barrelled, high velocity cannon. ACHZARIT PUMA StuG III

In all cases a very robust logistics organization is going to be needed to push supplies forward through contested ground and retrieve killed and injured troops and damaged equipment. Getting meals forward will be a huge chore when every corner could expose the logistics troops to sniper fire or an ambush. "Loggies" will also need to be adept fighting men, in addition to doing their usual duties on the front. (the deep rear troops like clerks and third line maintenance will probably be deeply resented "REMF's").

As for the character of the militaries, much will depend on the owning polity.

A polity which is mostly interested in protecting itself and not in conquering may develop a universal citizen militia on the Swiss model, where everyone has training and everyone carries and keeps military grade hardware at home or on their person. Invaders will not meet a conventional coordinated defense, but literally have to fight house to house as every single person will be capable of fighting and will be protecting their own personal property. A small permanent force might exist to use technical devices like AFV's, electronic warfare or protect from chemical and biological attack, and a skilled engineering force will exist to reinforce and rebuild structures both during an assault and after hostilities cease.

A polity which is interested in conquest will invest in legions of troops, well equipped and disciplined, capable of going into these deadly fights with as much manpower and equipment as necessary to overcome resistance. Only fairly large polities can afford the manpower and resources to do so, so only a few "empires" will exist in this setting.

Smaller polities which feel the need to project forces may evolve a more commando like force of raiders who can swoop down the tunnels and deliver a swift blow to knock their opponents back o their heels before the enemy can react. Once again, depending on the setting we can be talking anything from Ninja's to SoF forces. Given the restricted access through tunnels, they will mostly operate under cover to get in and out of targets. Soviet Era GRU "Spetsnaz" troops were often concealed as truck drivers for long range trucking firms, crewmen on merchant ships, communications troops in headquarters units (every HQ has lots of signallers) and embassy personnel, to give you an idea.

And of course every polity in this setting will have large and effective intelligence units busy placing spies and agents in every other city, in order to get up to date maps, profiles of leading citizens and military commanders, economic data and so on. The real war will be going on continually as intelligence forces battle counterintelligence forces in the shadows between actual hostilities.

Answer 2

One thing to keep in mind here is that you've given defenders a massive advantage. Having choke points that can be easily fortified and where forces can be concentrated is a defender's dream.

In contrast to the top voted answer (which I up voted) I argue that actually army sizes are going to make much less difference than it does in our world. If only 10 people can get down a tunnel at once then whether you have 100 people or 100,000 people behind them makes much less difference than if the 100,000 can swarm you from all directions.

Invasion would focus on stealth tactics to capture the defensive points by surprise or on digging new tunnels to bypass them. Defense would consist of multiple levels of defenses each arranged to deal as much damage as possible and culminating in completely collapsing the tunnel (ideally with the invading force still inside it).

"special forces" would be a really big deal here. If one special forces unit can capture a chokepoint and let your army flood into the city then they would achieve more than doubling the size of your army would.

Answer 3

Weapons and warfare will focus on tunnel defense

Since the tunnels are the only way to attack another city, busting through tunnel defenses and creating tunnel defenses will occupy the majority of a commander's time.

Military Organization

Much the same as medieval militaries. The king is commander in chief supported by a hierarchy of nobility, followed by mounted troops and infantry. Exactly how the military is organized beyond that rough hierarchy will greatly depend on the city, its resources, political culture and available armor/weapons.

Internal threats

The king will also need to worry about internal threats such as insurrections or rebellions that his military will need to deal with. The size and make up of these forces will depend on the size, culture and disposition of the city's inhabitants. Heavily equipped inhabitants will require a correspondingly heavily equipped military/police force to counter them.

Heavy infantry or light mounted cavalry would be sufficient in this case to maintain order. Note, that you don't need the horses for mobility across open ground but for the added bulk/power that a horse brings to a situation.

External threats

Since travel between cities must be done exclusively through these tunnels, the ability to choke them off will greatly strengthen the defenders hand. Even if rapid tunnel digging is possible, doing so is a noisy endeavor which is sure to alert the defenders.

Feasability of Extreme Distance Tunnel Building

Assuming that the average human can walk 30 miles a day, a two week trip is 420 miles. Note, that even with modern earth moving equipment, tunnels even a tenth as long as described the OP are very very rare (Thankfully the Ancients had magic to help them with the tunnels). Maintaining these tunnels would be an exceptionally expensive endeavor with cave-ins, dead-air pockets and vermin infestations.

Answer 4

Clearly, if you need to go through tunnels to invade a city, some of the military would be tunnel builders. This is identical to medieval times where it was not uncommon to use tunnelers to either build a tunnel into the enemy city, or to get a wall to collapse. The defenders also built tunnels to enter the attacker's tunnels and collapse them.

Sword and shield, club and shield, axe and shield, spear and shield, etc were common all around the world so it's safe to say these would still be used to fight in the tunnels and cities.

The usual risks also apply, such as mercenaries who are tasked with defending a city turning against the city once they are inside, a city being starved due to a prolonged siege, hidden agents poisoning the wells of the city, etc.

Answer 5

Weapons

I would suggest bows would be much less prevalent, there would be no need to hunt and the only bows would be specialist bow units who can be sent on top of walls/houses to shoot advancing enemy troops. The majority of soldiers would fight with short range weapons which are easy to use and without to much risk of them getting trapped in tunnels. Short swords and small shields with a few axes for breaking down doors/wooden walls. A couple of short spears may be used but probably not pikes or halberds as they would be hard to use in cities and tunnels.

Tactics

The main defensive tactic would be to blockade, barricade or trap the major tunnels then surround smaller tunnels so only one or two enemies can enter at once. The best offensive tactic would be to mine side tunnels so you could bring men up in unexpected places. A good one would be to come up in a leaders house and kidnap them. You would then surround or block the defenders in a small area and then kill or capture them.

Edit after reading material limits

Scrap the bows, they are too hard to make without wood, I would use slings instead. In ancient times slings were respected elite fighting weapons so it is feasible as a weapon of was, also portable so a soldier could have slings and swords. Spears and axes should have bone handles or metal handles. Shields could be bundles of reeds wrapped in leather, this would be o.k. but quite heavy and not brilliantly durable so maybe shields would be used less and they would use perhaps a padded sleeve on one arm to turn away blows.

Answer 6

Watchtowers :

Put watchtowers in every city, they provide a military advantage especially inside the city's walls :

A watchtower is a type of fortification used in many parts of the world. It differs from a regular tower in that its primary use is military, soldiers and from a turret in that it is usually a freestanding structure. Its main purpose is to provide a high, safe place from which a sentinel or guard may observe the surrounding area.