Troops fight in war because their leadership attempts to achieve something with violence. The exact mechanism how the effort is supposed to function depends on the specific circumstances and is often unknown in advance. People simply get used to the idea that sometimes you get what you want when you become violent.
This organized violence can vary a lot in its extent. Nation states with air, land and sea armed services can have the potential to wage the full range of organized violence. No power has ever been able to maximize its repertoire of organized violence to 100%, though. Judging by guts I'd say no power ever reached a greater capability than 90% of the possible repertoire. The Germans and Soviets of WW2 were unable of carrier warfare while the British and Americans were unable of certain tactics, for example.
No power exploits its full repertoire in war. There are always some capabilities in war that are considered to be too inadequate by themselves. Much more interesting is that the opposing power suppresses the use of additional capabilities. The British attempted to bomb Germany at daylight in 1940, but gave the idea up for the next years because losses were catastrophic and the effect marginal. The Germans didn't attempt any major offensives in Russia after Operation Zitadelle because no major offensive was promising any more.
The ability of armies to counter each other's capabilities is of greatest importance because it protracts warfare. Both powers' forces could simply advance into each other and come to a quick conclusion of the war as experienced in early Hellenic Polis Warfare. That doesn't happen any more because the option of simply advancing and attacking is in general suicidal in modern warfare; exceptions prove the rule and are called "successful offensives". The capability to simply advance & attack still exists, but it has been countered to such a degree that it's rarely a useful part of the repertoire any more.
This suppression of enemy capabilities can extend to defensive capabilities. At some point even a nation state army isn't capable of defending and holding terrain any more and needs to withdraw because it cannot match its opponent's capabilities any more. This happens usually not long before the state's collapse as a warring power.
A great geographic distance between battlefield and the homeland can still protract the war, of course.
The point of a state military's defeat is remarkably similar to the starting point of guerrillas. Occasionally, both are even historically matched as in the recent case of Iraq. Guerrillas are from the beginning unable to match most of their enemy's capabilities. They survive for a simple and extremely valuable advantage: The are elusive. Guerrillas are almost indistinguishable from civilians, so they can in fact survive without actually controlling any terrain.
The suppression of their capabilities is what coins the guerrilla war. Some guerrillas have enough capabilities to take out entire army garrisons or to control remote areas. Others are barely able to plant explosives and assassinate traitors.
The suppression of their capabilities has - just as the suppression of an opposing military's capabilities - a declining marginal rate. The addition of the same amount of resources offers ever smaller reductions of the guerrilla's useful repertoire.
The usual approach to conventional inter-state warfare - overpower your enemy - doesn't work that well against guerrillas. The latter do not reach the point of collapse so easily - they are already beyond it. They keep surviving thanks to their elusiveness. In worst case they could become sleepers and reduce their activities to a very low level. A level like mere terrorism, for example.
Meanwhile their opponent still needs to spend great resources to keep the guerrilla suppressed.
A counter-intuitive, yet promising move is to do something that's likely to be associated with failure and weakness. An army could allow the guerrillas to expand their useful repertoire instead of suppressing it as much as possible. The guerrillas might eventually step over a threshold and turn into a rather conventional force. Once beyond that point, it would be possible to push them back beyond that point - exactly what's being done in inter-military warfare to provoke a collapse. The result tends to be quite the same as in inter-military warfare: Collapse.
Musings about a military theory framework
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