During the night of 31 August, Nazi SS troops dressed themselves in Polish uniforms and staged an attack on the Gleiwitz radio tower in Upper Silesia. This ‘false flag’ operation was part of a wider series of staged attacks on Germans and German property called Operation Himmler that was designed to make it appear as if Poland was exercising aggression against Germany.

Just hours after the Gleiwitz incident, at 4.45am on 1 September, the first of approximately 1.5 million German troops launched their attack on Poland. This led to the encirclement of Polish forces thanks to the launch of coordinated attacks from the north, south and west. The attack from the south came across the border with Slovakia, which had declared its independence in March under pressure from Hitler.

Known as the Battle of the Border, Germany's three-pronged ground attack was supported by air raids that targeted key Polish cities. It took just 5 days for German troops on the ground to force the Polish army to retreat to their secondary defensive lines. The German government had already ensured that the Soviet Union would not respond aggressively to the invasion by signing the Nazi-Soviet Pact a week earlier. In accordance with the ‘secret’ protocol’ of this agreement, the USSR launched its own invasion of Poland from the east on 17 September, and this crushed Polish hopes of victory.

The Polish government refused to surrender to Germany and instead evacuated the country to form a government-in-exile in Allied countries.