In 1939 Stalin signed a nonaggression agreement with Adolf Hitler, the leader of Nazi Germany. He was persuaded of Hitler's virtue and disregarded the warnings from his military commanders that Germany was assembling their armies on its Eastern front. When Hitler struck in June 1941, the Soviet Army was completely vulnerable and they then suffered extensive losses. The Germans were then turned at Stalingrad in 1943. Stalin met with President Roosevelt of the United States and Prime Minister Churchill to deliberate postwar preparations. He demanded that the Allies should initiate a second front against Germany, which they then agreed to in the spring of 1944. Once again the three leaders met in February 1945. In this conference Stalin agreed to enter the war against Japan when Germany was defeated.