Description of the event

The campaign began on September 1st, 1939, one week after the signing of the secret Hitler-Stalin pact.
German forces invaded Poland’s western, southern and northern borders. Defending the long borders, the Polish armies were soon forced to withdraw east. The “Blitzkrieg” against Poland was one of several regional limited wars. As Germany did not react to take the troups back England and France declared Germany the war.
After the mid-September Polish defeat in the battle of Bzura, Germans gained undisputed advantage.
Polish forces began a withdrawal south-east, following a plan that called for a long defence in the Romanian bridgehead area where Polish forces were to await an expected Western Allies counterattack and relief.
On September 17th, the Soviet Red Army invaded the eastern regions of Poland.
The Soviets were acting in co-operation with Nazi Germany, carrying out their part of the secret appendix of the Molotov-Ribbentrop Pact (the division of Europe into Nazi and Soviet spheres of influence), which secured Hitler’s right flank, and allowed him to concentrate on attacking the Allies, without worrying about Soviet Russia to the east.
In view of this unexpected Soviet aggression, Polish government and its high command decided that the defence of the Romanian bridgehead was no longer feasible and ordered the evacuation of all troops to neutral Romania.
By the beginning of October, Germany and the Soviet Union had completely overrun Poland. Polish government, which never surrendered, together with many of its remaining land and air forces, successfully evacuated to neighboring Romania and Hungary.
Many of the evacuees subsequently joined the recreated Polish army in allied France, France-mandated Syria and the United Kingdom. Exactly in London, Polish government in exile was re-formed.