On the morning of September 1, waves of German bombers hit the railroads and hopelessly damaged the Polish mobilization. In four days, two armies; one on the north out of East Prussia, the other on the south out of Silesia had broken through the narrow fronts oh Poland and were sending armored spearheads on fast drives toward Warsaw and Brest. This was called the blitzkrieg (lightning war in German). The Germans used the military tactics of armor, air power, and mobile infantry in a pincers movement to encircle the Polish army.
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 deeper encirclement closed 100 miles near Brest. On that day, the Soviet Red Army quickly crossed the border. By September 20, practically the whole country was in German or Soviet hands, in the Soviet-occupied area, many thousands of Poles were deported to Siberia and many others were killed. Only isolated cities continued to fight and resist. The last to surrender was the fortress at Kock, on October 6. The estimated total of civilian casualties numbered more than 5 million, most of which was caused by the Germans. Polish military casualties in the war totaled about 600,000.