On Sept. 1, 1939, Adolf Hitler began Nazi Germany's invasion of Poland and ignited World War II. Hitler's invasion was calculated on the gamble that Western powers would not declare war in response to his invasion. While this proved inaccurate, Hitler had other motives for invading Poland. These include a Nazi-Soviet pact that had previously agreed to partition the country and the fact that Germany wanted its pre-World War I territory that was lost to Poland.

Nazi-Soviet Partition

In the summer of 1939, Hitler negotiated a treaty with the Soviet Union in which the two countries agreed to partition Poland. While the Nazi-Soviet Non-Aggression Pact was overtly a 10-year agreement not to go to war with each other, a secret protocol added both country's partition of Poland. Hitler's motivations for this treaty stemmed from a previous Anglo-French statement where those two nations agreed to defend the integrity of the Polish state from foreign aggression. Hitler thought a joint German-Soviet war in Poland would deter any Anglo-French action.

Lebensraum

Nazi Germany's ideology revolved partly around the idea that the German people needed more "lebensraum," or "living room." This idea stipulated that the racially superior German people would conquer their Slavic neighbors and enslave them. Before the invasion of Poland, Hitler had successfully annexed Czechoslovakia. Hitler desired to conquer all the Slavs, and so he invaded Poland to add to his conquest of Slavic nations.

Treaty of Versailles

The terms of the Treaty of Versailles that ended World War I was a prime motivator for German expansion into Poland. The treaty was widely viewed as unnecessarily oppressive in Germany, and many Germans harbored resentment against it. In the years before World War II, Germany had won back territory lost to France as a result of the treaty. This included Germany's reclaiming of the Rhineland in 1936 and its reunion with Austria in 1938, both of which were forbidden by Versailles. Likewise, Hitler wanted Polish territories lost in the treaty, such as West Prussia, Poznan and Upper Silesia.

Hitler's Miscalculation

Hitler's invasion of Poland partly rested on the calculation that the Western powers -- mainly France and Great Britain -- would not declare war in response to Germany's invasion of Poland. This proved inaccurate, and both countries declared war on Nazi Germany two days after the invasion to ignite World War II. In June of 1941, Germany broke its nonaggression pact with the Soviets, and declared war. In the process, Germany seized the parts of Poland previously acquired by the Soviets, and the World War expanded.