World War II first Attack: On 1 September 1939, Germany and Slovakia a client state in 1939 attacked Poland. On 3 September 1939 after Germany failed to withdraw in accordance with French and British demands, France and Britain, followed by the countries of the Commonwealth, declared war on Germany but provided little military support to Poland other than a small French attack into the Saarland. On 17 September 1939, after signing a nonaggression pact with Japan, the Soviets launched their own invasion of Poland. By early October, Poland was divided among Germany, the Soviet Union, Lithuania and Slovakia, although Poland never officially surrendered and continued the fight outside its borders. At the same time as the battle in Poland, Japan launched its first attack against Changsha, a strategically important Chinese city, but was repulsed by late September.

Following the invasion of Poland and a German-Soviet treaty governing Lithuania, the Soviet Union forced the Baltic countries to allow it to station Soviet troops in their countries under pacts of "mutual assistance." Finland rejected territorial demands and was invaded by the Soviet Union in November 1939. The resulting conflict ended in March 1940 with Finnish concessions. France and the United Kingdom, treating the Soviet attack on Finland as tantamount to entering the war on the side of the Germans, responded to the Soviet invasion by supporting the USSR's expulsion from the League of Nations. In June 1940, the Soviet Armed Forces invaded and occupied the neutral Baltic States.

In Western Europe, British troops deployed to the Continent, but in a phase nicknamed the Phoney War by the British and "Sitzkrieg" (sitting war) by the Germans, neither side launched major operations against the other until April 1940. The Soviet Union and Germany entered a trade pact in February of 1940, pursuant to which the Soviets received German military and industrial equipment in exchange for supplying raw materials to Germany to help circumvent a British blockade.

In April 1940, Germany invaded Denmark and Norway to secure shipments of iron ore from Sweden, which the Allies would try to disrupt. Denmark immediately capitulated, and despite Allied support, Norway was conquered within two months. British discontent over the Norwegian campaign led to the replacement of Prime Minister Neville Chamberlain by Winston Churchill on 10 May 1940.

The German army attacking Poland is the first battle on World War II.