On 1 September 1939, 62 German divisions supported by 1,300 aircraft began the invasion of Poland. At 8pm on the same day, Poland requested military assistance from Britain and France. Two days later, in fulfilment of their April 1939 pledge to support the country in the event of an attack, Britain and France declared war on Germany. World War II had begun.
The Anglo-French declaration of war may have been unexpected, but Hitler's prediction that the campaign against Poland would be short and victorious was correct.
At 6am on 1 September, as Warsaw was battered by the first in a succession of bombing raids, two German army groups invaded Poland from Slovakia in the south and Prussia in the north. The German air force, which had much more advanced aircrafts than the Poles, quickly established air supremacy by attacking and destroying the Polish air force in the air and on the ground. This allowed German bombers to attack road and rail junctions, as well as concentrations of Polish troops. Towns and villages were bombed to spread terror among civilians and generate a fleeing mass of refugees which would block the roads and prevent reinforcements from arriving at the front. Junkers Ju-87 dive-bombers destroyed any strong points in the German path. By 8 September, German tanks were already on the outskirts of Warsaw. A week later, the capital was completely surrounded.
A Polish counterattack on 9 September led to the Battle of Bzura, the largest engagement of the campaign. Despite some limited early Polish success, massive German air superiority, and their ability to quickly redirect forces to meet the Polish attack, led to a crushing German victory.
Poland had been overrun in four weeks, long before any meaningful Anglo-French military aid could reach Poland, and proving Hitler's conviction that Polish armed forces would be no match for the Blitzkrieg ('lightening war') unleashed by Germany.
The Soviet Union – which had spent the 1930s searching for a 'collective security' agreement in Europe and had signed a non-aggression pact with Germany on 28 August 1939 – also profited from the invasion. On 17 September the Red Army crossed the Polish border in the east, seizing a third of all Polish territory. Some Poles fled across the border into Romania, eventually reaching the west and continuing the war as the Free Polish Forces. Warsaw finally surrendered at 2.00pm on 27 September.