In less than two weeks, Hitler planned to invade Poland, but with Britain and France guaranteeing Polish independence, he needed to ensure that the Soviet Union would not side with the West in the war. Officially, the Nazis hated Communists, but Hitler now needed to work out a deal with the supreme leader of the Soviet Union, Joseph Stalin .
Ribbentrop met with Stalin and the Soviet foreign minister, Vyacheslav Molotov, and negotiated a pact that would change the lives of millions of people and reshape the boundaries of much of eastern Europe. In the public section of the final document they signed, they agreed that the two countries would not go to war with each other or support any country at war with the other. They also agreed to increase trade between their countries. But the document also had secret clauses that gave each country "spheres of influence," or future control, over distinct sections of eastern Europe. Stalin demanded control over territories that had been controlled by czarist Russia and lost in World War I, including the entire state of Latvia. As for Poland, the river Vistula would divide the country, with Germany claiming the western territory and the USSR occupying the eastern portion. The agreement was set to last 10 years.
The non-aggression pact, signed on August 24 by the two foreign ministers, became known as the Molotov-Ribbentrop pact. Eight days later, on September 1, 1939, Germany launched a massive attack on Poland from the west with 1,500 tanks, 1,500 airplanes, and 1,500,000 men. On September 3, Britain and France declared war on Germany; Britain proceeded to bomb some German naval bases but took little other direct action. On September 17, more than 600,000 Red Army troops invaded Poland from the east and headed toward the agreed-upon demarcation line. Within days, the Red Army occupied half of Poland.
Near the end of the month, Ribbentrop made a second trip to the Soviet Union to meet with Stalin and Molotov again. The men discussed securing their empires in eastern Europe. At this meeting, Stalin strongly pledged his country’s support for Germany, something he never admitted after becoming allied with the West in 1941.
“If, against all expectation, Germany finds itself in a difficult situation then she can be sure that the Soviet people will come to Germany's aid and will not allow Germany to be strangled. The Soviet Union wants to see a strong Germany and we will not allow Germany to be thrown to the ground.”- Joseph Stalin, 1939
In London, the British government had no intention of declaring war on the Soviet Union, having already declared war on Germany. On September 20, Prime Minister Neville Chamberlain told the House of Commons, "What we will not do is to rush into adventures that offer little prospect of success."
The United States provided more good news for Stalin. Though President Franklin D. Roosevelt despised the Nazis and the Communist Soviet Union, he had to answer to the American public who wanted to stay out of another European War at any cost. The United States declared its neutrality.
With little to fear from the West, the Soviets happily collaborated with the Nazis. Together they built a new border between them in the place where Poland had once been, and the German Gestapo and Soviet secret police, the NKVD, held meetings to see how they could best cooperate.