1. They can't, at least not as plainly as stated here. The wars being fought today are asymmetrical and, in many cases, ideological. You can take out a pocket of resistance here or there, but there are no huge battles like in a war between two nations. Victories are small, cost more, are not decisive, but are guaranteed due to the overwhelming strength of the weapons.
2. More importantly: they don't want to. long drawn out wars are profitable, numbing, and demoralizing. Most people don't care about them anymore. They just happen in the background. No thumbs up, no thumbs down. Go on about your business. That's the best type of reaction a government could ask for when pushing the agenda of lobbyists and the corporations they represent.
People will beg the government to come in and save them from themselves.
So the question, I suppose, is what could trigger the actual occupation? My guess is that it will be austerity measures that will come through widespread municipal bankruptcies. Pensions and disability checks will stop coming. Services will stop being provided.
There is a whole section of the population that subsists on government provided benefits. Multiple generations, below the poverty line, directed acyclical graphs of income where the last person to have worked is a generation or two ago. When that lifeline goes away, then we will see unrest. Then the government will come in and occupy, providing basics. Give us your guns and we will protect you. Take this rice and this water and go home and watch TV.
> there are no huge battles like in a war between two nations
Nor need there be! That's not how guerrilla or "asymmetrical" warfare works; the chief advantage for the guerrillas is that they disappear into the populace between attacks, so there's no one for the more ordinary sort of army to have a battle with.
The way you win a war like this, as the occupier, is to counter this advantage in a way that makes guerrilla warfare proportionately costly for the guerrillas, which ordinarily it is not. For example, you might respond to guerrilla attacks by taking hostages from among the populace, and if the attackers fail to surrender, proceeding to execute the hostages taken. While hardly nice, this method is very effective, and can eventually produce the total and complete pacification of the territory under occupation. (Of course, so can mass relocation or genocide, but those are absurdly expensive and time-consuming by comparison, not to mention far more costly in terms of political will, and are thus best reserved for cases where hostage-taking doesn't work, such as the North American aborigines in the early 19th century, or the Armenians in the early 20th.)
You can achieve victory in an ordinary war by blowing things up, more or less. In a guerrilla war, more subtle means are required. And nastier means, no doubt! -- I don't think anyone has ever tried to claim that military occupation and the forcible curtailment of guerrilla warfare were nice, and I'm certainly not making any such claim right now. I'm just pointing out that, on the one hand, this is what you have to do to win, and on the other hand, it is possible to win a military occupation, if the political will exists for it to be competently carried out.
In recent American history, of course, such will never does exist, because for all the generic American ignorance of history, foreign affairs, and military matters, we do at least have the basic good sense to recognize that engaging in military occupation, of a country which never did us harm and isn't about to start, is both wrong and expensively pointless. That's why it takes stupid, venal, dishonest leaders to fool us into supporting such adventures.
(Of course, it helped a lot that after Vietnam such leaders stopped conscripting the children of the mostly progressive elite, which did a great deal to mute progressive opposition when the Bush II claque came along; instead of finding some way to suppress people whose political opinions actually mattered, the Iraq adventurists could count on a volunteer military in which the elite had no significant personal stake, thus no significant reason to try to prevent being squandered. But that's a different discussion altogether.)
Would they? They should try doing that now in all the various wars and police actions they are involved in.