The Polish army expected the attack to come along the Polish frontiers. But Hitler introduced a new kind of war called blitzkrieg.

In the early morning hours of September 1, 1939, the German armies marched into Poland. Man for man, the German and Polish forces were an even match. Hitler committed about 1.5 million troops, and the Polish commander, Marshal Edward Smigly-Rydz, expected to muster 1.8 million. That was not the whole picture, however. The Germans had six panzer (armored) and four motorized divisions; the Poles had one armored and one motorized brigade and a few tank battalions. The Germans' 1600 aircraft were mostly of the latest types. Half of the Poles' 935 planes were obsolete.

Polish strategic doctrine called for a rigid defense of the whole frontier and anticipated several weeks of preliminary skirmishing. It was wrong on both counts.

On the morning of September 1, waves of German bombers hit the railroads and hopelessly snarled the Polish mobilization. In four more days, two army groups—one on the north out of East Prussia, the other on the south out of Silesia—had broken through on relatively narrow fronts and were sending armored spearheads on fast drives toward Warsaw and Brest. This was blitzkrieg (lightning war): the use of armor, air power, and mobile infantry in a pincers movement to encircle the enemy.

Between September 8 and 10, the Germans closed in on Warsaw from the north and south, trapping the Polish forces west of the capital. On September 17, a second, deeper encirclement closed 160 km (100 mi) east, near Brest. On that day, too, the Soviet Red Army lunged across the border. By September 20, practically the whole country was in German or Soviet hands, and only isolated pockets continued to resist. The last to surrender was the fortress at Kock, on October 6.