All of them, obviously.
Whether they could or not, that's a different question.
That wasent the question. I have revised my post.
Well, that's very different. In my response I was assuming whether any of those wars in general should've been avoided (ie, the Germans should've avoided letting that extremist fuck take the country to war). Strictly from the US point of view, the answer would be different, but since I already voted on the poll I'll just go through these one by one.
War of 1812 - US had a pretty solid motive for war, an illegal blockade on it's trade, but used it as an opportunity to expand into Canada and break resistance from the Indians in order to expand more easily westerwards. Could've been avoidable, but seems like it was kind of legitimate.
Civil War - This one is tough, on the one hand, the US had absolutely no right to go to war with secessionists, while on the other the secessionists were just
really bad. In other words, the war was basically illegitimate, but was historically vindicated by leading to the abolition of slavery (perhaps the most disgusting institution ever engaged in by man, even if that was not the real pretext for it which it was fought). Should it have been avoided? It's iffy, but probably not.
Spanish American War - this War was nothing but a propaganda coup for yellow journalism. The Americans literally used and manipulated the Cuban, Puerto Rican, and Filipino independence movement and crafted an empire of their own out of Spain's rotting carcass. Totally useless and avoidable.
WWI - In this one, the United States sent troops to defend a couple of big trading partners from a war they were not entirely free of blame from causing, in order to get a spoil or two. Not particularly necessary, should've probably been avoided, but world history would be
much different that's for sure.
WWII - This one's kind of a no-brainer. US gets attacked by Japan (though the Japanese sort of had good reason to). The European theater is debatable but, I mean, c'mon...
it was against Hitler for chrissakes.
Korean War - This one was pretty legit. Pretty clear [UN Charter] "Chapter VII" affair. (Country gets invaded, international community responds)
Vietnam War - This one was just stupid. Blackmailed by the French, the Americans undertake the effort of propping up a corrupt sub-colonial dictatorship that's hated by virtually all Vietnamese, alienating and then fighting the anti-colonial liberation movement. The result was a Communist Vietnam, millions of Vietnamese dead, and 60K dead Americans. Moronic and useless.
Gulf War - Another clear Chapter VII violation of the UN charter, this one is kind of legit.
Iraq War - An illegal war of aggression started under false pretenses for virtually
no reason. This one speaks for itself.
Afghanistan War - This one's kind of silly. Deceivingly framed as "self-defense," the US decides to try to kill a band of insane fanatical thugs by
bombing and occupying an entire country. Whoa there guys, relax - some clever intel and a single special-ops team might be enough... and it was. Should've been avoided.
So we here got 5 that could've should've would've been avoided (Spanish, WWI, Vietnam, Iraq, Afghanistan), 3 that could've been avoided but were kinda justified (1812, Korea, Gulf War), and 2 that were kinda unavoidable (Civil, WWII).