The invasion of Poland, 1 (September – 6 October 1939), was a joint attack on the Republic of Poland by Nazi Germany, the Slovak Republic, and the Soviet Union, which marked the beginning of World War II. The invasion began on 1 September 1939 when Germany marched into Poland, preceded by the Danzig crisis and by the Molotov–Ribbentrop Pact between Germany and the Soviet Union, which was signed one week before and approved by the Supreme Soviet of the Soviet Union one day before the invasion. On 17 September the Soviet Union invaded as well, acting on a secret protocol of the pact, which assigned parts of Poland to them. The campaign ended on 6 October with Germany and the Soviet Union dividing the whole of Poland under the terms of the German–Soviet Frontier Treaty.
The aim of the invasion was to disestablish Poland as a sovereign country, with its citizens destined for extermination. German and Slovak forces invaded Poland from the north, south, and west the morning after the Gleiwitz incident.
As the Wehrmacht advanced, Polish forces withdrew from their forward bases of operation close to the Germany–Poland border to more established defense lines to the east. After the mid-September Polish defeat in the Battle of the Bzura, the Germans gained an undisputed advantage. Polish forces then withdrew to the southeast where they prepared for a long defence of the Romanian Bridgehead and awaited expected support and relief from France and the United Kingdom.
On 3 September, based on their alliance agreements with Poland, the United Kingdom and France declared war on Germany; in the end, their aid to Poland was very limited. France invaded a small part of Germany in the Saar Offensive, and the Polish army was effectively defeated even before the British Expeditionary Force could be transported to continental Europe.
On 17 September, the Soviet Red Army invaded Eastern Poland, the territory beyond the Curzon Line that fell into the Soviet "sphere of influence" according to the secret protocol of the Molotov–Ribbentrop Pact; this rendered the Polish plan of defence obsolete.[20] Facing a second front, the Polish government concluded the defence of the Romanian Bridgehead was no longer feasible and ordered an emergency evacuation of all troops to neutral Romania.[21] On 6 October, following the Polish defeat at the Battle of Kock, German and Soviet forces gained full control over Poland. The success of the invasion marked the end of the Second Polish Republic, though Poland never formally surrendered. Source-Wikipedia
