Also a comment said there was a US Civil Peace flag with basically inverted colors, but the only link I found was kind of shady. Anyone know anything about that?
There was a similar thread about a year ago: http://www.reddit.com/r/AskHistorians/comments/22xub7/its_often_stated_that_the_us_has_been_at_war_for/
The executive summary is that you have to be extremely generous with the term "war" for this to be anywhere close to accurate. It's complicated since the U.S. has only declared war a handful of times but was involved in conflicts that we would certainly consider to be wars.