Pacifists say, "Absolutely nothing." Their slogan is "War Is Not the Answer," for all times and all places. It's a lame slogan, if a well-meant one, on which younger right-wingers have zeroed in, looking for an easy target. Protest Warrior claims that the group invented a counter-slogan:
Copperheads or on the Confederate side in the era of the Civil War in the United States, and who would have supported Franco in the Spanish Civil War, can't take credit for solving slavery, fascism, and Nazism. Besides, what put an end to Communism (aka state socialism) wasn't war but the decision of the Communist Party elite (see David M. Kotz and Fred Weir, Revolution from Above: The Demise of the Soviet System, Routledge, 1997).
But it is a little sad to think that, among slavery, fascism, Nazism, and Communism, only the last vanished from the pages of history without being militarily defeated by an external enemy. State socialism created its own grave-diggers, the bureaucratic power elite, but capitalism hasn't. Historical materialism must account for this problem and propose a new solution.