Hostile militaristic dictatorships with unchecked power will eventually be my problem or my children's problem or their children's problem, I would like to try to nip the problem in the bud. Of course, if the USA were to invade and conquer Russia right now it would create whole new problems of civil rights and complete lack of representation just like the Pacific Islands and Puerto Rico but at a massive scale, but that's in my opinion still better than the risk of allowing them to expand and grow in power to the point where nothing can be done to stop them.
Same reason I would have signed up to fight nazis. Authoritarianism is always worth fighting.
Inb4 someone says "the us army is authoritarian," as if the inherent hierarchy in an army is in any way equivalent to the violations of civilian rights that actual authoritarian regimes commit