In a civilization with modern to near-future technology, are there ways so that a siege against fortresses, and eventually against cities itself, would still be required to properly conquer said city, or even be the most effective method? Are there ways to reduce the effectiveness of tactics in this setting that do not involve soldiers actively conducting siege warfare similar to the medieval era?
Siege is still an element of modern warfare. If a faction wants to get in a city and another one doesn't want to let them in, a siege is the natural consequence.
We have had some famous examples in the recent years, just to cite a couple:
- The battle of Stalingrad
The Battle of Stalingrad (23 August 1942 – 2 February 1943) was the largest confrontation of World War II, in which Germany and its allies fought the Soviet Union for control of the city of Stalingrad (now Volgograd) in Southern Russia.
- Siege of Leningrad
The siege of Leningrad [...] started on 8 September 1941, when the Wehrmacht severed the last road to the city. Although Soviet forces managed to open a narrow land corridor to the city on 18 January 1943, the Red Army did not lift the siege until 27 January 1944, 872 days after it began. The blockade became one of the longest and most destructive sieges in history, and possibly the costliest in casualties suffered.
- Siege of Sarajevo
The Siege of Sarajevo was the siege of the capital of Bosnia and Herzegovina, and the longest of a capital city in the history of modern warfare. After being initially besieged by the forces of the Yugoslav People's Army, Sarajevo was besieged by the Army of Republika Srpska from 5 April 1992 to 29 February 1996 (1,425 days) during the Bosnian War. The siege lasted three times longer than the Battle of Stalingrad and more than a year longer than the Siege of Leningrad.
As additional info on what is so difficult with siege, look at the siege of Montecassino: as long as the abbey was a no fight zone, German troops were stationed outside, and were a relatively easy target. Once the abbey was bombed and became a ruin, it became a wonderful hiding place for the German troops, who could hide and attack with much more ease.
In principle, a siege is a strategy with the express goal of defeating your opponent through slowly attritting their forces.
With modern warfare’s application of combined arms — the tactical use of land, air, and naval units simultaneously to achieve a tactical goal — the pace of conflict is so fast that battles are either decisive and fast or forces keep their distant from each other.
But, Sieges are still relevant today in conflicts involving cities with civilian populations where the attacking force wants to take the city more or less intact without killing or at least minimizing the civilian population.
If the defending forces value the civilian population, they might abandon the city knowing the attacking units aren’t going to harm the civilians left behind. So no siege since the defenders would leave before they were encircled. The Russian invasion of Georgia can be interpreted as an example of this situation
If the defending forces don’t value the civilian population, then they can use them as human shields or as hostages. This causes the attacking forces to move slowly, house by house and street by street. The recent news showed this kind of warfare in the The Siege of Aleppo, and other battles in the Syrian.
I guess the same arguments apply to fortresses. But, unless a fortified area was needed to be kept intact, I would think attackers would destroy it with air power. And, conversely, if there was something in the fortress or the location of the fortress was in a key location to support future attacks on their opponents, then the attacker might try to take the fortress intact despite the expectation of high casualties on both sides.
Presuming the attacking force has modern warfare of even today, a siege requires that there be some reason the attackers don't want to just obliterate the defenders. Or that they are having a hard time doing that.
If the attackers simply want to wipe out the defenders they can do this in short order. They can drop a huge variety of chemical explosives from fuel air weapons to bunker busters. Any ordinary building with ordinary walls, doors, windows, etc., will go to the fuel air weapon. Any hard target with walls less than 6 meters thick will fall to a bunker buster. Even if the defenders are not killed outright, they will be buried under many meters of rubble.
If it's some extreme situation there is even the possibility of going nuclear. It would probably only happen if the attackers were somehow a coalition of nearly every nation in the world, and the defenders somehow drastically offensive to nearly everybody in the world. Offhand I can't think of a candidate. But it could happen. As a science fiction angle, maybe it's invading aliens and they have vile habits.
There must be some reason the attackers want to be relatively selective.
Perhaps there is a civilian population that is relatively uninvolved, and the attackers don't want to appear to be monsters for killing them wholesale. For example, the intended targets might be some small group of individuals that would be perceived as legitimate targets. Maybe terrorists launching missiles from a suburb. Or a defeated leadership after a war could have retreated to some stronghold in the middle of an otherwise pacified civilian population. There is desire to get the targets but not wipe out large numbers of non-combatant civilians.
Perhaps there is some value in the defense site such as historical monuments or buildings, famous works of art. Maybe it's a museum with many thousand works of art. Maybe it's a famed base of a religion, such as a major church or shrine.
Maybe the defenders have some other resource the attackers don't want to simply wipe out. The only son of the president of the attackers, for example.
Maybe the defenders have some suicide option. They have a bunch of bombs placed at key locations around the world, and if they get smashed to little bits the hidden bombs get set off.
Or, to go all science-fiction on you (since you added that tag) the defendants might have some counter measure.
Maybe they've got extra hardened bunkers. Perhaps they've got the local equivalent of a really good metal smith, and their bunkers simply shrug off the bombs. Or maybe they have some super tunneling ability, and they can hide 100 meters under the ground and pop up 2 km from the bombed location, take some shots, and hide again.
Maybe they've got some really good anti-aircraft gear. Maybe two or three decades of research on the Iron Dome has resulted in something that makes it very hard to hit a protected target with planes or missiles. Though in the quite near future that could probably be pretty much brushed aside. Project Thor would see orbital kinetic weapons hitting the target at mach 10. It's quite a challenge to know what would stop that. It's unlikely that even a laser based defense could stop that, at least with next-couple-decades tech.
As to methods of siege, modern tech has provided lots of options. Depending on the nature of the target. Of course there will be some kind of surrounding to prevent additional resources getting in. There will be attacks on command and control such as electronics, radio equipment, etc. There will be attacks on any vehicles. There will be attempts at infiltration, to get intel, to open defenses, to perform sabotage, and to attempt to convince the defenders to give up.
We have also seen quite a few innovations recently. Playing loud music 24 hours a day to keep the defenders from sleeping. Chemical irritants that nudge right up to but don't cross limitations on chemical warfare. For example, dropping canisters of rotten egg gas. Bright search lights and laser beams shinning into the defender areas, both to obscure what the attackers are doing and just to be irritating. Radio noise to keep them from communicating. Enough electronic noise can even stop commercial grade computers from operating.
And depending on the nature of the defenders there may well be lots of other leverage. Do the defenders have relatives outside the defense location? Do they care about some religious site? Do they have financial holdings they are hoping to use after they get away from the siege? All of these could be threatened to good psychological advantage.
By using the loose definition: siege - cutting off an enemy's supplies and lines of communication.
Then a siege makes military sense at all times, periods and epochs.
Once your enemy is not fully self-sufficient, you gain from cutting him off.
A siege will look different at various times. A siege will also vary in scale, depending on the period.
Judging from recent history sieging is a problem. Yugoslavian war had almost 4 years long siege of Sarajevo.
Siege for Aleppo in Syria took time between 2011 and 2016 and although government stated that they control whole city there are still some fights on the outskirts.
Main problem with sieging is that you might want something from that particular place. It might have industry, control over port, banks or information, technology or the place is good for hiding and need to be control for defence purposes (like Tora Bora).
With modern and near-future technology you could limit the amount of soldiers. Self-driving drones, artillery that could be operated with limited staff. But you would still need some foot on the ground as the everchanging landscape of sieged city/fortress would require human-like abilities to recognize, adapt and react.
Your question seems to me to be en reality two questions:
1- why anyone would want to capture a city
2- why just not nuke it, assault it and going forward.
First cities are often node in central communication networks, railway and large road go through cities. Those communications assets are very important for an army in order to maintain logistic supply. you just cannot make thousands of trucks going each day on a road free setting they will just make the path impracticable. (The fortress of Brest is a good example of a fortress that controls communication line)
Second, they are an area of population concentration. A city not controlled can be a danger for an army. But it can also represent relief for an army were to rest. In much long term, most value produced by a country is from cities (except natural extraction heavy resource). Capturing a city is capturing population and economic assets.
Some cities also have political significance. And strategic assets such as political building, electricity generator, factory, airports.
If you a city do not represent a strategic, a political or tactical value for an army it happened that an army will siege it to avoid being an attack from it, but will use existing alternative communication lines. This has been used by American in the pacific for example, they did not capture-recapture every island only those they needed to reach japan and secure their line of communication or those with political interest such as the Philippines.
Now, why besieging a city when you can storm it. They are several factors to considers:
1- Can you really storm the fortress/cities? You can look of WW1 or even WW2 fortress that endures incredible punishment and hold for months, noteworthy Osewiec fortress and Verdun. A mistake here could be really costly The german think they could storm Stalingrad.
2- If you can storm the fortress what do this will cost you in term of manpower and material? don't you risk to lose valuable ressources? don't you risk to destroy valuable asset of the cities?
3- How many time do you have? For the Brest fortress, his capture was imperative because the german army planned to use those communication lines. For Leningrad, they decided to have time and did not want to risk a potentially costly assault.
4- The particular nature of a city made it a very tough combat zone, limited visibility, civils (hostile, friendly or worst unknown). Cities are places where you can lay ambush and trap, where a group of men can delay an unarmored column for hours using building cover.
If the defender is determined, and have enough force left to give a fight you can expect it to be very tough and costly combat.
They are two way of besieging a city: Both imply to isolate the city from logistic. (no more freshmen, no more food, no more water, no electricity)
Then you have the times you can wait to starve to death, that can take some time you have an example of the success of such operation with the siege of Siege of La ROchelle; To work you need to cut off food and men supply totally.
The second option you don't have time, or you want to take it back ASAP but you cannot afford to storm it. This is what happened at Mosul, first, the city was circumvented, and then using support (artillery, air, drones, armoured vehicles) the troops made slow progress to gain small tactical gain. But even so, well-determined foes were able to slow their progress and inflicted significant causalities.
You seem to also ask what can make a siege long IMO it is large supply or capacity to bring supply despite the circumvent of the city. And determination soldiers in siege live a miserable life, to endure it and to fight they need to be highly determined.
I hope my answer covers your question. I did not cover much the political aspect of it.
Technically, the fight with the Taliban, Al-Qaeda, ISIS, and other terrorist groups have included several sieges, if the whole wars aren't sieges in themselves.
When Osama bin Ladin was hiding in caves, Allied forces were in siege mode. They tried to prevent supplies from entering the underground tunnels, prevented reinforcements from entering & exiting, and didn't simply just destroy the tunnels with large ordinance. Part of that is because of how fortified the opposing forces were (since they were using mountains as strong points), but also because they wanted to minimize civilian or other non-combatant casualties.
In all reality, those forces had access to missiles that could have been fired into the cave openings and detonated far into the cave, causing a shock wave that would have killed everything in those caves. Allied forces did use some of these missiles, but not as often as they could have.
American and Allied forces have essentially occupied Iraq and Afghanistan for over a decade. We are working with those governments to ferret out terrorist groups, even going door to door to try to get rid of them. We have set up blockades at entry points in various cities, as well as fought against opposing forces in walled palaces, waiting for them to run out of food, water, and ammunition so we can either dramatically enter with little opposition remaining or they eventually surrender.
While this has had large and small scale battles throughout the War on Terror, it's still had the effect of cutting off supplies and reinforcements to terrorist groups. At the same time, over the past several years, we've also been rebuilding what we've blown up or tore down.
We've also replaced what the terrorist groups destroyed. Before Saddam Hussein took power, Iraq was at about equivalent to 1950-60's American culture, with cars, movies, clothes, education, and more. After he got into power, he sucked the region dry to accumulate his wealth, so not all of the damage was done by warfare, but I'm getting off the subject here.
So yes, sieges still have a tactical use in warfare. They allow a more gradual overtaking of a city than just flattening it.
Sieges exist in modern times because while there is no fortress wall that the attackers can't get past defending infantry shooting from ambush in a city is extremely deadly to the attackers. Taking a city by force of arms is a bloodbath that will cause big troubles back home for the invader.
That leaves either flattening it from the air (bombs or artillery) which will also cause considerable upset, or a siege.
For near-future flattening it from the air might not even be a viable approach.
Sieges make sense when:
- The location is a MacGuffin, so bypassing it is useless or impossible, or
- Some person(s) in the location need to be captured alive, or
- Some thing(s) in the location need to be captured intact
- It is possible to both hide deep enough to survive artillery or bombardment, but get back from shelters to fighting positions fast enough to resist assaults that occur immediately after bombardment ceases, and
- It is much easier to resist a frontal assault than successfully attempt a frontal assault, and
- There are critical supplies (such as water, food, fuel, ammunition, or mana) that can run out, and
- It is possible to stockpile the critical supplies, and
- Counters (such as bazookas or rocket propelled grenades) exist that can be used in tight quarters against sturdy offensive weapons (such as tanks), and
- It is possible to defend against poison- and germ-warfare attacks, and
- It is possible to defend against oxygen-deprivation attacks, and
- "Nuking the site from orbit" is "off the table".
These criteria set up conditions where infantry are required to successfully advance through the target area to root out sheltered fighters, but where frontal assault is impractical. A siege can force the defenders to use up critical supplies, so that the final assault is against a much-weakened defense.