From Howard Zinn, “Just and unjust wars”, transcript from a public talk, as found in Failure to quit (Haymarket books, 1993):
Einstein was horrified at World War I, as so many were, that great war for democracy, for freedom, to end all wars, etc. Ten million men die on the battlefield in World War I and nobody at the end of it understands why, what for. World War I gave war a bad name until World War II came along.
Einstein […] devoted a lot of time to thinking and worrying about it. He went to this conference in Geneva. He thought they were discussing disarmament, to do away with the weapons of war and therefore to prevent war. Instead, he found these representatives of various countries discussing what kinds of weapons would be suitable and what kind of weapons needed to be prohibited. What were good weapons and bad weapons, just weapons and unjust weapons? Einstein did something which nobody ever expected. He was a very private man. He did something really uncharacteristic: he called a press conference. The whole international press came, because Einstein was, well, he was Einstein. They came, and he told this press conference how horrified he was by what he had heard at the international conference. He said, “One does not make wars less likely by formulating rules of warfare. War cannot be humanized. It can only be abolished”.