War is not particularly complex, it's just not something liberals are usually willing to understand as it challenges their little mythologies (many of which you repeated here).

Relatively simple questions are unanswerable by that framework, not even approximately. Let's try some.

• Who pushes for war in the first place? Where does the impetus come from? Normal folks don't wake up and say, "yeah I'd like to destroy a country and its people 4500 miles away", and they definitely don't have the power to make war happen.

• What gains the consent of the country to support and maintain war? Why do anti-war movements, even with millions of people, fail to stop war?

• Why do the wars end? When they achieve their purpose? What purpose was that?

• Who benefits from the wars? Are they involved in the process?

Of course, the driving factors here are simply capitalism and its political lackeys, attacking from multiple angles to ensure its seat of empire will achieve the desired ends by pushing and by removing obstacles. The impetus is a series of battling foreign policy think tanks, politicians ready to support military spending, a friendly (and racist!) media apparatus, and war profiting companies paying every single one of those groups to keep the heat up for the next boondoggle. Constant vilification of established "enemies" and attempts to create new ones, usually targeted at countries that undermine the power of the global seat of capital and therefore its ability to exploit labor and resources internationally.

This is why Saudi Arabia is an "ally" while Iran is an enemy. All things the same, Americans would be just as racist towards both, care just as little for their lives, know just as little about them. But one cozies up to the hegemony of international capital and the other does not, so you are to hate the one and not the other. Scads of anti-Iranian think tanks and propaganda while the Saudis get occasional mention and can even murder journalists on US soil and get away with it. It's not actually that complex so long as you don't believe lies about American democracy, "freedom", interest in peace, liberal world order, etc.

So when we know that these are the actors and criteria, why some wars and not others? Why not big new wars every 6 months instead of several years? Well, the interests involved are part of global capital, they respond to the rate of profit and crises of capitalism, and politicians are on their side. Both the capitalists and their buddies in Congress know that war is a "stimulus" and they count it as jobs and profits and campaign donations (legalized bribery) and good press. The opportune time is whenever it can be sentimentally capitalized on, whenever they can get away with it. When it's hurting the "right people" at the time, where they might have to wait for consent to get manufactured first. When times are tough and "jobs" mean particularly more than other people's lives.

And more deeply and perniciously, capitalism forms society itself, such as the white supremacist settler culture of the United States where it is never that difficult to whip up support against another ethnicity, just requires jumping through a few different hoops depending in which capitalist party you favor. The intense gullibility and susceptibility to propaganda, in part due to schools' materials being dictated by reactionary school systems that themselves work in concert with large publishers to create verifiably false and simplistic material into history textbooks, lesson plans, etc (see: Texas' input on other states' curricula). The precarity forced on so many that they can't even consider joining an anti-war movement. The normalization of American military violence and widespread societal myths about its impact, its actual activities, its history.

I don't think any of this is complicated. It is only uncomfortable for some.