On August 31, 1939 German forces dressed up as Poles seized the German radio station in Gliewitz Upper Silesia, they were establishing an excuse to invade Poland. The next day on September 1, German forces began their invasion of Poland. The Polish forces were incapable of stopping the German forces which used the technique that became known as the blitzkrieg, an integration of armor forces air support with traditional infantry. The Poles expected that the Allies would come to their immediate defense. Both France and Great Britain declared war on Germany but did not attack.

Polish forces rapidly withdrew from their front line forward defensives to their second line further East. By September 8th German forces reached the outskirts of Warsaw, and assumed that they there was no major operational army capable of counter attacking. Most of the Polish army was still in the field and had merged together and decided to counterattack . The Germans were overstretched and the initial Polish attack which had come as a surprise was successful. Polish forces advanced nearly 20 miles in the area west of Warsaw near the Bzura River. The German reacted with most of their 10th Army which included over 800 tanks and attempted to surround the attacking Polish forces. Most of the Polish troops were soon trapped. The counteroffensive succeeded in slowing down the German forces which were forced to react and change their battle plans. However on September 19th when the Soviets invaded Poland from the East and chance of continuing to defend Polish territory became irrelevant and the surviving Polish army units crossed into Romania and eventually joined the Allied armies.