The Invasion of Poland in 1939 was a military offensive in which Nazi Germany and, two weeks later, the Soviet Union, invaded Poland. It was the start of World War II in Europe. The invasion took place from 1 September to 6 October 1939. The invasion of Poland caused Britain and France to declare war on Germany on 3 September; they did little to affect the September Campaign. In the end, Poland lost. Germany and the Soviet Union divided the country according to a treaty signed years before the war.

It is often called the first time that blitzkrieg was tried on the battlefield, but similar methods had been used earlier. The German surprise attack was successful. It was very effective against the ineffective and demobilized Polish Army, whose tanks and airplanes were nearly out and mostly old. They were outflanked, outmaneuvered, and outnumbered in September 1939, and easily destroyed by the Blitzkrieg. The Poles, if well prepared, could have had two million soldiers in the fight.

In the first few days of the German invasion, the Poles proved to be more difficult than the Germans expected. In the Battle of Mokra on September 2nd, the Poles repulsed an attack by a German Panzer division forcing their retreat. The German air force, unlike their invasions of other countries, did not easily overcome the outnumbered Polish Air Force. However, due to their great superiority in numbers, they gained air superiority after 5 to 6 days.

Surrounded by German territories north, west, and south of Poland, the Poles had little space for tactical retreat. The Germans reached Warsaw and attacked on September 7 but were repulsed. They had suffered heavy tank losses and so were forced to retreat. The Battle of Bzura, on September 9, was the largest attack by the Poles during the war. The Poles had great initial success by decimating 2 German divisions and taking 3500 German prisoners. Polish infantry fought well, but by September 19, overwhelming German reinforcements defeated the Poles.

The Poles realized the British and French were in a phoney war and had no intention of assisting Poland, and the Red Army invaded Poland from the east on September 17, the Polish high command ordered as many troops and equipment to the Romanian bridgehead to enter Romania with the intention to continue their resistance by moving as much of their army to France as possible. Most of the Polish Navy escaped to Britain. Polish cryptographers who had cracked the German Enigma (machine) went to France and then to Britain to continue their work.

Polish politicians set up a government-in-exile in France and later escaped to Britain. In 1945, when the Allied armies invaded Germany, forcing their surrender, the Polish army was the fourth largest allied army.