Organized nongovernmental peace movements began in 1815 in New York City. In 1843 the first international peace congress met in London, England. By 1914 there were about 160 peace societies in the world. Some, such as the Carnegie Endowment for International Peace, have been heavily subsidized and intensely active in working for international goodwill; but most have been affiliated with governments or international power blocs and have gone along with them when war threatened or erupted.

This has generally been the case of the world's Socialist parties, which were established on what was thought to be the bedrock of international brotherhood and “the parliament of the world.” The United States Socialist leader Eugene V. Debs went to jail during World War I for interfering with recruitment for the armed forces. Running for president from his prison cell, he received 1 million votes. That event, however, was the peak or past the peak of truly international Socialism.

At the outbreak of World War I, the French antiwar Socialist leader Jean Jaurès was assassinated, and Socialist members of both the French and the German parliaments voted war credits to their respective governments. Today's Socialists or as they call themselves, Social Democrats have almost uniformly abandoned pacifism. They have come to power from time to time in England, France, the Scandinavian countries, and post-Nazi Germany.