The Comintern held seven World Congresses in Moscow between 1919 and 1935. During that period it also conducted thirteen "Enlarged Plenums" of its governing Executive Committee, which had much the same function as the somewhat larger and more grandiose Congresses. The Comintern was officially dissolved by Joseph Stalin in 1943 to avoid antagonizing its allies the United States and the United Kingdom.
While the differences had been evident for decades, World War I proved the issue that finally divided the revolutionary and reformist wings of the workers' movement. The socialist movement had been historically antimilitarist and internationalist, and therefore opposed workers serving as "cannon fodder" for the "bourgeois" governments at war. This especially since the Triple Alliance (1882) comprised two empires, while the Triple Entente gathered France and Britain into an alliance with Russia. Karl Marx's The Communist Manifesto had stated that "the working class has no country" and exclaimed "Proletarians of all countries, unite!" Massive majorities voted in favor of resolutions for the Second International to call upon the international working class to resist war if it were declared.
Nevertheless, within hours of the declarations of war, almost all the socialist parties of the combatant states announced their support for the war. The only exceptions were the socialist parties of the Balkans and the British Labour Party. To Lenin's surprise, even the Social Democratic Party of Germany (SPD) voted in favor of war credits. The assassination of French socialist Jean Jaurès on July 31, 1914 killed the last hope of peace by removing one of the few leaders who possessed enough influence on the international socialist movement to prevent it from segmenting itself along national lines and supporting governments of National Unity.
Socialist parties in neutral countries mostly supported neutrality rather than total opposition to the war. On the other hand, during the 1915 Zimmerwald Conference, Lenin organized opposition to the "imperialist war" into a movement that became known as the "Zimmerwald Left" and published the pamphlet Socialism and War, in which he called all socialists who collaborated with their national governments "Social-Chauvinists", that is, socialists in word but chauvinists in deed.
The International divided into a revolutionary left and a reformist right, with a center group wavering between those poles. Lenin condemned much of the center as social-pacifists for several reasons, including their voting for war credits despite opposing the war. Lenin's term "social-pacifist" aimed in particular at Ramsay MacDonald, leader of the Independent Labour Party in Britain, who opposed the war on grounds of pacifism, but did not actively resist it.