Adolf Hitler wanted more land, especially in the east, to expand Germany according to the Nazi policy of lebensraum. Hitler used the harsh limitations that were set against Germany in the Versailles Treaty as a pretext for Germany's right to acquire land where German-speaking people lived. Germany successfully used this reasoning to envelop two entire countries without starting a war.
Austria: On March 13, 1938, Germany took over Austria (termed the Anschluss) - a contingeny specifically disallowed in the Versailles Treaty.
Czechoslovakia: At the Munich Conference on September 28-29, 1938, the French and the British handed Germany a large portion of Czechoslovakia. Hitler then took the rest of Czechoslovakia by March 1939.
Many people have wondered why Germany was allowed to take over both Austria and Czechoslovakia without a fight. The simple reason is that Great Britain and France did not want to repeat the bloodshed of World War I. They believed, wrongly as it turned out, they could avoid another world war by appeasing Hitler with a few concessions (such as Austria and Czechoslovakia). At this time, Great Britain and France did not understood that Hitler's goal of land acquisition was much, much larger than any one country.
After having gained both Austria and Czechoslovakia, Hitler was confident that he could again move east, this time acquiring Poland without having to fight Britain or France. (To eliminate the possibility of the Soviet Union fighting if Poland were attacked, Hitler made a pact with the Soviet Union - the Nazi-Soviet Non-Aggression Pact.)
So that Germany did not officially seem the aggressor (which it was), Hitler needed an excuse for attacking Poland. It was Heinrich Himmler who came up with the idea; thus the plan was code-named Operation Himmler.
On the night of August 31, 1939, Nazis took an unknown prisoner from one of their concentration camps, dressed him in a Polish uniform, took him to the town of Gleiwitz (on the border of Poland and Germany), and then shot him. The staged scene with the dead prisoner dressed in a Polish uniform was supposed to appear as a Polish attack against a German radio station.
Hitler used this staged attack