While political science has probed the ideological, political, economic and even psychological motivations behind the drive to war, socialist theory has made a unique contribution by highlighting the relationship between the development of capitalism and war. There’s a long and rich tradition of the Left’s opposition to militarism that dates back to the International Working Men’s Association. It is an excellent resource for understanding the origins of war under capitalism and helping leftists maintain our clear opposition to it. In this talk, Marcello Musto examines the position of all the main currents (socialist, socialdemocratic, communist, anarchist and feminist) intellectuals (Engels, Kropotkin, Malatesta, Jaurès, Luxemburg, Lenin, Mao and Khrushchev) of the Left on the war and its different declinations (‘war of defence’, ‘just war’, ‘revolutionary war’).