At 4:45 a.m., some 1.5 million German troops invade Poland all along its 1,750-mile border with German-controlled territory. Simultaneously, the German Luftwaffe bombed Polish airfields, and German warships and U-boats attacked Polish naval forces in the Baltic Sea. Nazi leader Adolf Hitler claimed the massive invasion was a defensive action, but Britain and France were not convinced. On September 3, they declared war on Germany, initiating World War II.
The Nazis intended to destroy the Polish nation completely. In 1941, the Nazi leadership decided that Poland was to be fully cleared of ethnic Poles within 10 to 20 years and settled by German colonists. From the beginning of the occupation, Germany's policy was to plunder and exploit Polish territory, turning it into a giant concentration camp for Poles who were to be eventually exterminated as "Untermenschen"