 Although General Secretary Joseph Stalin had signed the Non-Intervention Agreement, the Soviet Union contravened the League of Nations embargo by providing material assistance to the Republican forces, becoming their only source of major weapons.Unlike Hitler and Mussolini, Stalin tried to do this covertly.Estimates of material provided by the USSR to the Republicans vary between 634 and 806 aircraft, 331 and 362 tanks and 1,034 to 1,895 artillery pieces.