In Never at War, Spencer Weart shows that in the modern era, democracies are extremely unlikely to fight other democracies, but that non-democracies fight democracies, and each other, much more often. You can try to think of counterexamples (Mexican-American War, maybe) but you'll be straining to do so.
A new paper by Blank, Dincecco and Zhukov show that prior to the modern era (i.e., 1200-1800) parliamentary states were actually MORE likely to go to war than absolutist states were. Their argument is that parliamentary states were successful in that they actually had more capacity to make war than the absolutist states, but they had not yet developed institutional constraints to prevent them from doing so, as presumably occurred in the modern era. (H/T slatestarcodex)