War breaks out in Europe

Common parade of GermanWehrmacht and Soviet Red Armyon 23 September 1939 in Brest,Eastern Poland at the end of the Invasion of Poland. At centre is Major General Heinz Guderian and at right is Brigadier Semyon Krivoshein.

On 1 September 1939, Germany and Slovakia—a client state in 1939—attacked Poland. On 3 September 1939 France and Britain, followed by the countries of theCommonwealth, declared war on Germany but provided little support to Poland other than a small French attack into the Saarland. On 17 September 1939, after signing anonaggression pact with Japan, the Soviets also invaded Poland. Though Poland was divided by Germany, the Soviet Union, Lithuania and Slovakia; the Poles didn't surrender and established a Polish Underground State and the insurgent Home Army, and continued to fight on Allied fronts outside Poland. In the Romanian Bridgehead operation, some 120,000 Polish troops were evacuated to France, along with much of the Polish Air Force and Poland's Enigma codebreakers. During this time, 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.

German troops by the Arc de Triomphe,Paris, after the 1940 fall of France.

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 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 were about to disrupt. Denmark immediately capitulated, and despite Allied support, Norway was conquered within two months. In May 1940Britain invaded Iceland. British discontent over the Norwegian campaign led to the replacement of Prime Minister Neville Chamberlain by Winston Churchill on 10 May 1940.