On September 1st., 1939, 1.8 million German troops invaded Poland on three fronts; East Prussia in the north, Germany in the west and Slovakia in the south. They had 2600 tanks against the Polish 180, and over 2000 aircraft against the Polish 420. Their "Blitzkrieg" tactics, coupled with their bombing of defenceless towns and refugees, had never been seen before and, at first, caught the Poles off-guard. By September 14th. Warsaw was surrounded. At this stage the poles reacted, holding off the Germans at Kutno and regrouping behind the Wisla (Vistula) and Bzura rivers. Although Britain and France declared war on September 3rd. the Poles received no help - yet it had been agreed that the Poles should fight a defensive campaign for only 2 weeks during which time the Allies could get their forces together and attack from the west.

On September 17th. Soviet forces invaded from the east. Warsaw surrendered 2 weeks later, the garrison on the Hel peninsula surrendered on October 2nd., and the Polesie Defence group, after fighting on two fronts against both German and Soviet forces, surrendered on October 5th. The Poles had held on for twice as long as had been expected and had done more damage to the Germans than the combined British and French forces were to do in 1940. The Germans lost 50,000 men, 697 planes and 993 tanks and armoured cars.
Source: http://www.kasprzyk.demon.co.uk/www/history/WW2.html