After Vietnam the US Army said - We're not doing that again.
After WW1 the Brits and the Germans said - We're not doing that again.
The French persisted with a classical defence based on the Maginot Line and the Brits and the Germans said - See we told you so.
The Russians persisted in throwing bullet sponges at machine guns along with lots and lots of really big bullets of their own.
Meanwhile the Navy, operating on dead flat terrain that could be easily undermined by sappers in submarines and overflown by bombardiers in aircraft, and forced to stay on the move, had to adopt a 4 dimensional form of war that balanced the offence and the defence. They built a form of warfare out of umbrellas and bubbles.
My concern has been that the Western way of war on land has not been adequately addressing the need for the defence. To me it is as if we have given into the belief that we will always have secure bases of operation from which we can sally at our leisure and engage whomever we choose whenever and wherever it suits us.
But we have bases. We need bases and bases will always be vulnerable. One of the most ridiculous treaties that anybody ever devised was the Anti-Ballistic Missile treaty. We chose Mutually Assured Destruction over applying technology to defend against inanimate objects. Reagan's Star Wars was a stretch but now we are talking about C-RAM systems on every vehicle to knock anything that flies out of the sky. Reagan wasn't wrong. He just needed more time than he thought it would take. I believe the reason the Russians were so opposed to Star Wars is that they had no faith in their own ability to generate a comparable system. They had been importing other people's ideas since Peter the Great. Their missiles were the spoils of war and their bombs were stolen.
That is why I have been mumbling from time to time about "Frigates in the Desert". Every frigate, every sub, every carrier, task force, flotilla or fleet has to configure for all round defence all the time. Even when alongside the pier. It uses any available technology to sense everything possible. It has short, medium and long range systems to counter any threat from any aspect. It has ordnance to every purpose and its entire war is about figuring out how to bring that ordnance to bear while dodging enemy ordnance.
By contrast the army's war is dominated by imposed geometry. It operates in two dimensions on a surface. Its limits are the front, the rear and the flanks.
The subsurface war of sappers and tunnels and mines is an aberration to be dealt with on occasion. Generally speaking their has been little need to worry about what is underfoot - but that is no longer true. Toe-poppers are everywhere - laws of armed conflict be damned.
The overhead war is post 1914 phenomenon that waxed and waned because of the cost of putting bombs in the air and dropping them accurately. But now every body with a video game controller and a toy glider can be dropping HE accurately on your individual head.
The cavalry force, whether squadron or three corps army now stands revealed as a very expensive, slow moving projectile, constrained by geography and vulnerable to attack from above and below.
It also requires a firm base from which to operate and the bigger the force that bigger the base it requires and the slower it moves making it ever more vulnerable to attacks from above, below and behind. And the farther it advances the longer its lines of communications grow and the more resources it has to commit to ever defending ever increasing vulnerabilities.
@markppcli helpfully referenced US FM 7-7. In there the M113 is described as a mobile base for the squad. It was a secure storage facility for the squad that could take the squad anywhere, including over creeks and small rivers and keep them supplied for 72 hours of operations and allowed them to fight unencumbered knowing that their water, a hot meal and a warm steeping bag were right behind them. It also, like every other base, created a target which required defending. A target that was all the more valuable if the base's "garrison", the M113's squad, were all tucked up in side. The M113 was more secure when the garrison manned the battlements, supplying air guards, and its artillery, the 50, was manned and ready.
Conversely the M113 was a less valuable target when the garrison had sallied forth .
Every base needs an active defence. It needs a sally force. But it also benefits from ordnance on the battlements, ordnance that can provide bubbles and umbrellas of protection. And for fixed bases, bases that will always be occupied, ports and airports, towns, cities and countries, then the need for a permanent ordnance based umbrella or bubble becomes critical.
And the sally force, emanating from those garrisons, large force or small force, like the navy's frigates, will have to takes its bubbles along with it.
I know that people laugh at my mediaeval and ancient references. So be it. The reason I do is because I don't think the elements of struggle have changed since people retreated to caves and lit a fire to let the family get a good night's sleep. I am willing to bet that somebody with a sharp pointy stick stayed awake staring into the dark wondering if he was going to have to go out and meet the enemy or was the enemy going to come to him.
Ordnance, gunpowder in particular, was the application of technology to the defence of the realm. It made it possible to leave more people catching fish, growing crops and making plows rather than having to man the battlements. It also kept the enemy at bay, with the safe area increasing with the range of the cannons. At least until the enemy dragged their own cannons in range. But if you can see the enemy coming and your cannons can outrange their cannons, or you can bring your cannon's into action before they are in range then the threat is managed and eliminated.
Manoeuver is part of warfare. But there is an irreducible need for a solid, static defence.