On 1 September 1939, Germany and Slovakia—a client state in 1939—attacked Poland. On 3 September France and Britain, followed by the countries of the Commonwealth, declared war on Germany but provided little support to Poland other than a small French attack into the Saarland. Britain and France also began a naval blockade of Germany on 3 September which aimed to damage the country's economy and war effort.
On 17 September, after signing a cease-fire with Japan, the Soviets also invaded Poland. Poland's territory was divided between Germany and the Soviet Union, with Lithuania and Slovakia also receiving small shares. The Poles did not surrender; they established a Polish Underground State and an underground Home Army, and continued to fight with the Allies on all fronts outside Poland.
About 100,000 Polish military personnel were evacuated to Romania and the Baltic countries; many of these soldiers later fought against the Germans in other theatres of the war. Poland's Enigma codebreakers were also evacuated to France. 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.
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 the Allied 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 1940 Britain invaded Iceland to preempt a possible German invasion of the island. British discontent over the Norwegian campaign led to the replacement of Prime Minister Neville Chamberlain with Winston Churchill on 10 May 1940.