September 1 - At around 04:00 the German battleship Schleswig-Holstein, anchored in the port of Gdansk, opens fire on the nearby Polish naval station of Westerplatte: it is the beginning of the second world war. At 04.45 German troops cross the border while the Luftwaffe attacks Polish military bases and installations: the Polish campaign begins.

The United Kingdom orders mobilization and prepares evacuation in anticipation of air strikes.

Italy declares its "non-belligerency" but new laws establish a war economy: ban on selling meat on some days of the week, prohibition of using iron and reinforced concrete in private buildings, coal rationing for heating, tests of electrical dimming.

Estonia, Finland, Latvia, Lithuania, Norway and Switzerland declare their neutrality.

The Republic of China and the Japanese Empire are involved in the early stages of the third year of armed conflict between them during the Second Sino-Japanese War. The war is in what will be known as the "Second Period", which begins in October 1938 and ends in December 1941. At the time the conflict merged into the world war: Japan joined the Axis in September 1940, China at Allies at the end of December 1941.

2 September - Adolf Hitler declares that he will respect the territorial integrity of Norway, provided that it is not "threatened or violated by other countries".

The United Kingdom and France issue a common ultimatum for Germany, which requires German troops to evacuate Polish territory; Benito Mussolini declares Italian neutrality; President Douglas Hyde of the Republic of Ireland declares the neutrality of his nation, [without a source] the Swiss government orders the general mobilization of its forces.

The National Service Act is immediately enacted, requiring full conscription on all males between the ages of 18 and 41 living in the United Kingdom.

The Free City of Gdansk is annexed to Germany. Hardy entrenched in the city's post offices are overwhelmed.