<P> The bulk of Soviet fighting took place on the Eastern Front--including a continued war with Finland--but it also invaded Iran (August 1941) in cooperation with the British and late in the war attacked Japan (August 1945), with which the Soviets had border wars earlier up until in 1939 . </P> <P> Stalin met with Winston Churchill and Franklin D. Roosevelt at the Tehran Conference and began to discuss a two - front war against Germany and the future of Europe after the war . Berlin finally fell in April 1945 . Fending off the German invasion and pressing to victory in the East required a tremendous sacrifice by the Soviet Union, which suffered the highest military casualties in the war, losing more than 20 million men . </P> <P> In August 1939, Stalin accepted Hitler's proposal into a non-aggression pact with Germany, negotiated by the foreign ministers Vyacheslav Molotov for the Soviets and Joachim von Ribbentrop for the Germans . Officially a non-aggression treaty only, an appended secret protocol, also reached on 23 August, divided the whole of eastern Europe into German and Soviet spheres of influence . The USSR was promised the eastern part of Poland, then primarily populated by Ukrainians and Belarusians, in case of its dissolution, and Germany recognised Latvia, Estonia and Finland as parts of the Soviet sphere of influence, with Lithuania added in a second secret protocol in September 1939 . Another clause of the treaty was that Bessarabia, then part of Romania, was to be joined to the Moldovan SSR, and become the Moldovan SSR under control of Moscow . </P> <P> The pact was reached two days after the breakdown of Soviet military talks with British and French representatives in August 1939 over a potential Franco - Anglo - Soviet alliance . Political discussions had been suspended on 2 August, when Molotov stated that they could not be resumed until progress was made in military talks late in August, after the talks had stalled over guarantees for the Baltic states, while the military talks upon which Molotov insisted started on 11 August . At the same time, Germany--with whom the Soviets had started secret negotiations on 29 July--argued that it could offer the Soviets better terms than Britain and France, with Ribbentrop insisting, "there was no problem between the Baltic and the Black Sea that could not be solved between the two of us ." German officials stated that, unlike Britain, Germany could permit the Soviets to continue their developments unmolested, and that "there is one common element in the ideology of Germany, Italy and the Soviet Union: opposition to the capitalist democracies of the West". By that time, Molotov had obtained information regarding Anglo - German negotiations and a pessimistic report from the Soviet ambassador in France . </P>

Who was the leader of the soviet union in ww2