It's kind of a mix. The Vietnam War is mostly unpopular now, but at the time there were plenty of liberals foaming at the mouth to "stop communism". They may dislike the conservatives, but they're still nationalists, and being called communists by the republicans all the time has put a chip on their shoulder where they want to prove that they're both patriotic and smarter than them. The Iraq War is a good example: the liberals saw the massive genocidal war crime unfolding in front of them, and their response was "this is so awful and sloppy and senseless, Bush is uniquely stupid, vote for us and we'll do this war of occupation in a smart, sensible way."

They're mostly just as bloodthirsty as the overt right wingers, but they have some kind of decorum/denial fetish that compels them to do little procedural, bureaucratic and semantic dances around their bigotries and libidinal urges, rather than just admit them, even to themselves