On August 23, 1939, Nazi Germany and the Soviet Union signed the “Treaty of Non-aggression between Germany and the Union of Soviet Socialist Republics”, where both sides agreed not attack the other or be alliance to or assist the enemy of either side. The treaty became known in the West as the Molotov-Ribbentrop Pact, so named for the foreign ministers Vyacheslav Molotov of the Soviet Union and Joachim von Ribbentrop of Germany.
The treaty included a secret protocol that delineated each side’s spheres of influence in the regions between them: Poland, Romania, the Baltic states Estonia, Latvia and Lithuania, and Finland. The non-aggression treaty was made public, but the secret protocol became known only at the end of World War II from the German copy of the document found in Nazi archives. The treaty ended when Germany launched its invasion of the Soviet Union on June 22, 1941.
The treaty allowed Germany free rein to attack Poland on September 1, 1939, launching what would unexpectedly become the global World War II.
(Taken from Wars of the 20th Century – World War II in Europe)
Background to the German Invasion of Poland Britain and France, which had pursued appeasement toward Hitler, had become wary after the German occupation of the rest of Czechoslovakia, which had a non-ethnic German majority population, which was in contrast to what Hitler had said that he only wanted returned those German-populated territories. Britain and France were now determined to resist Germany diplomatically and resolve the crisis through firm negotiations. On March 31, 1939, Britain and France announced that they would “guarantee Polish independence” in case of foreign aggression. Since 1921, as per the Franco-Polish Military Alliance, France had pledged military assistance to Poland if that latter was attacked.
In fact, Hitler’s intentions on Poland was not only the return of lost German territories, but the elimination of the Polish state and annexation of Poland as part of Lebensraum (“living space”), German expansion into Eastern Europe and Russia. Lebensraum called for the eradication of the native populations in these conquered areas. For Poland specifically, on August 22, 1939 in the lead-up to the German invasion, Hitler had said that “the object of the war is … to kill without pity or mercy all men, women, and children of Polish descent or language. Only in this way can we obtain the living space we need.” In April 1939, Hitler instructed the German military High Command to begin preparations for an invasion of Poland, to be launched later in the summer. By May 1939, the German military had drawn up the invasion plan.
In May 1939, Britain and France held high-level talks with the Soviet Union regarding forming a tripartite military alliance against Germany, especially in light of the possible German invasion of Poland. These talks stalled, because Poland refused to allow Soviet forces into its territory in case Germany attacked. Unbeknown to Britain and France, the Soviet Union and Germany were also conducting (secret) separate talks regarding bilateral political, military, and economic concerns, which on August 23, 1939, led to the signing of a non-aggression treaty. This treaty, which was broadcast to the world and widely known as the Molotov Ribbentrop Pact (named after Soviet Foreign Minister Vyacheslav Molotov and German Foreign Minister Joachim von Ribbentrop), brought a radical shift to the European power balance, as Germany was now free to invade Poland without fear of Soviet reprisal. The pact also included a secret protocol where Poland, Finland, Estonia, Latvia, Lithuania, and Romania were divided into German and Soviet spheres of influence.
One day earlier, August 22, with the non-aggression treaty virtually assured, Hitler set the invasion date of Poland for August 26, 1939. On August 25, Hitler told the British ambassador that Britain must agree to the German demands on Poland, as the non-aggression pact freed Germany from facing a two-front war with major powers. But on that same day, Britain and Poland signed a mutual defense pact, which contained a secret clause where the British promised military assistance if Poland was attacked by Germany. This agreement, as well as British overtures that Britain and Poland were willing to restart the stalled talks with Germany, forced Hitler to abort the invasion set for the next day.