"some" of the oil fields. Most of them are run owned by anything but American companies, from China and Russia, so 20 years later, let's simply drop this myth about Americans invading countries for their resources as it's getting pretty old. I love that the fact that everyone is talking about the riches of Iraq and Afghanistan but none of the ones in Korea or Vietnam, maybe because it doesn't fit the narrative, right?
Then why the same mistake is being repeated, again and again? Did they stopped after Vietnam? Korea was different, because at that point almost all the cold war superpowers got involved. But Vietnam was an internal power struggle and the US had no business there. And take the example of Iraq. The justification given was that Saddam was manufacturing WMDs. Two decades have passed. Where are the so called WMDs? And then came Afghanistan. After that it was Libya. A country that was one of the most prosperous in the African continent is now even worse than Somalia.
There are several reasons, accurate or not:
First, the belief from many that stopping fighting somewhere else or at least confining it there can prevent it from spreading.
Second, that innocent people deserve help - e.g. to prevent things like the Holocaust, genocide etc. Look at the hundreds of millions that were killed by the statist authoritarians over the last century.
Third, the (perhaps real reason) deep state (per Glenn Greenwald in 2017 ) is invested in power and war. For example, look at who provided the "evidence" of WMDs? The three letter agencies (DoD, NSA, CIA, etc). Look who provided the "intelligence" about Afghanistan? Three letter agencies again. All unelected bureaucrats who have nothing to fear from being wrong for decades or committing perjury - Clapper and Brennan - about spying on the US (and the world) per Snowden. Some, like Clapper and Brennan don't believe in liberty, only power and will do much to undermine the US overseas in order to get people to distrust liberty.
Fourth, nation building - give people a taste of a republican form of government combined with democracy and they'll be able to keep it.
Number 1 and 2 are often used as public rationale while 3 is the deep rationale. Good, sincere people may believe in the first two while being used by those in 3.
Which are real, which are accurate, which make sense, I don't know, but those are some reasons about which I have read. Personally, I think a lot of people believe #2, and really mean it. They're being hijacked though.