September 1, 1939 was the day Hitler invaded Poland and the date on which World War II officially began. In 1934, Hitler signed a non-aggression pact with Poland. Many Germans wanted Germany to regain the land Germany lost to Poland after World War I in the Treaty of Versailles, including West Prussia and Upper Silesia. Hitler signed the nonaggression pact so Poland would not form an alliance with France, but his promises of nonaggression were manipulative, as he did not intend to keep them.
During the years following the nonaggression pact, Hitler began to remilitarize his country. In 1936, Germany occupied the Rhineland. Even after Hitler annexed Austria with the Anschluss in 1938 and took over parts of Czechoslovakia called the Sudetenland in 1938, the European powers practiced a policy of appeasement in the Munich Conference. They believed that, if they officially granted Hitler what he had already conquered, he would stop his rampage. Human nature led European powers to want to believe Hitler would end his aggression, but that was not the case. After the Munich Conference, Hitler violated what he had promised and took apart the Czech state. Again, he manipulated political power and went against what he promised.
On September 1, 1939, Hitler invaded Poland with over 2,000 tanks and more than 1,000 planes, crushing the Polish military. The Poles capitulated on September 27. The Russians, with whom Hitler had signed a nonaggression pact in 1939, attacked Poland from the east. Later, Hitler went against his nonaggression pact and attacked the Soviet Union in 1941.