The invasion commenced on 1 September 1939 taking the Poles completely by surprise. The Poles gave a good account of themselves in spite of the overwhelming odds. Stiff Polish resistance caused the Germans delays and some setbacks. A week into the campaign the Germans approached Warsaw and fighting there would be vicious and prolonged. The city did not surrender until 27 September after heavy aerial bombardment. On 17 September the Soviets invaded from the east against negligible resistance. The last Polish forces surrendered on 6 October, making it far longer than the "18-day campaign" claimed by German propagandists. In spite of mass surrenders, up to 100,000 Polish troops made it out of Poland into nearby countries to later fight on the side of the Allies in other theaters.

The Germans lost some 16,000 dead and 32,000 wounded. Over 670 tanks were knocked out with just over 200 being un-repairable. Over 66,000 Polish military were killed and 134,000 wounded. The Germans took 587,000 prisoners and the Soviets 200,000. This successful campaign prompted the Germans to look to the West and later face the French and British military.