Warning: cynical essay I started writing before anyone replied to this thread. There's a summary at the bottom if you aren't in a wall of text mood.
Do you know what a "Casus Belli" is? It means a reason to go to war. Its part of the idea that countries need a justification to go to war, and in recent history the aggressor in a war will typically provide a justification for why when they declare war.
This is not necessarily the actual reason, its more of a PR thing. In WW1, for example, the US Casus Beli was the sinking of the Lusitania by a German U-Boat. The president already wanted Britain to win and we were supporting them with arms, but the isolationist feelings at the time meant they needed a reason before they could declare war.
Now, look at how the US styles itself as world policeman. Look at how government talks about "defending democracy", even though historically we've supported democratic and non-democratic regimes equally. The USA, as with most countries, believes in a realistic (read: serving the interests of yourself, your friends, and your country, and nothing else) attitude towards politics. By playing up Enemies of the State such as dictators, non-democrats, non-capitalists, fundamentalist non-Christians, or terrorists, Congress and the President give themselves an unlimited Casus Belli on almost every country that isn't in Europe or East Asia. And the really great thing, from their perspective?
They don't have to use it. Most governments in the world have some characteristic the US considers objectionable, but we don't fight them. Yet as soon we stand anything to gain from fighting someone, all we have do is look on this list:
- Is its government communist or a dictatorship?
- Does it have any ties, real or imagined, to any terrorist or insurgent group?
- Is it small enough a certain amount of Marines could overthrow the government quickly with no one noticing? (this one is a loophole: no CB is needed, since its not war by our definition)
- Is it currently involved in a revolution, AND the government has a bad reputation?
- Is it in conflict with any of our allies? Note: new allies can be made for this one.
- Do terrorists have influence there?
- Does it have a WMD program, real or imagined?
- Are ANY amount of US citizens in danger there?
If ANY of those are true, congratulations, the USA can wage war on you whenever it wants, because its people are primed to accept that kind of war. Convincing them to continue supporting a war that doesn't truly affect them, that's a different story...
Now, I'm not saying our elected officials are always pragmatic bastards seeking some deeper interest. I honestly think they believe some of their own rhetoric. But the motive to maintain this mindset is there, and they can't possibly not be aware of that. I also believe that the whole Arab = terrorist, Muslim = terrorist, Israel = the noble kingdom we must defend thing has been played up by the government because they want to be able to pick their fights in the Middle East.
Now, what's the US government's interest in the Middle East? I... don't know. But there is a certain logic behind all recent US military actions. The thing is, we have an extremely powerful lobby group called the Military-Industrial Complex, that provides constant pressure to expand the military and that combined with a certain diehard military cheerleader faction of the Republican party makes it unfeasible to cut the military down. Yet are military is STUPIDLY LARGE. Ignoring things like nukes and focusing on pure numbers, if the entire world were to simultaneously wage war on the US, we would stand a chance. Seriously. Our military buildup is that large. And that costs money, loads and loads of money. So our pragmatic, selfish government (as most are) has this extremely expensive tool that they can't get rid off. So what do they do? Use it, whenever they have anything that even slightly needs fixing. Country not supporting our corporate or trade interests? Looks like there's some poor brown people that need to be liberated from their horrible dictator. One terrorist group exploited a flaw in our security to blow up two buildings and kill a bunch of people? Let's wage literal war on the entire concept of terrorists, as if terrorists had a country we could invade. Want... whatever it is we want in the Middle East (oil? Preventing a powerful rival state from forming? Pushing some religious or moral agenda? who knows)? Pick one of like 4 different things from the above list, we can basically fight any Arabic nation we want at this point.
As you might be able to tell, I disagree with my government on what our foreign and military policy should be, and and also have a cynical view on this issue. There are probably more optimistic viewpoints here. But to keep some distance I should mention that this is very historically common: a powerful nation pushes around weaker nations for its own interests. The US is not unique in this respect.
Also, there is an element of defensiveness to all this. The idea is we need to stay on top, counter rival world powers, or else we might eventually be invaded or strong-armed ourself.
tl;dr: Our military costs money so we need to use it. To use the military we need to declare war. To declare war we need a reason to war. To have a reason to war with as many countries as possible, we need to take advantage of the bad feelings many US citizens have for other worldviews. To do that, we need to dress up our wars as fighting nebulous evils such as "tyranny" or "terror".