<P> Between 1919 and 1939 Poland pursued a policy of balancing between the Soviet Union and Nazi Germany, seeking non-aggression treaties with both . In early 1939 Germany demanded that Poland join the Anti-Comintern Pact as a satellite state of Germany . Poland, fearing a loss of independence, refused, and Hitler told his generals on 23 May 1939 that the reason for invading Poland was not Danzig: "Danzig is not the issue at stake . It's a matter of extending our living space in the East ..." To deter Hitler, Britain and France announced that an invasion would mean war and tried to convince the Soviet Union to join in this deterrence . Moscow pretended to play along with this wish but found it could gain control of the Baltic states and parts of Poland by allying with Germany, which it did in August 1939 . London's deterrence had failed, but Hitler did not expect a wider war . Germany invaded Poland on September 1, 1939, and rejected the British and French demands that it withdraw, resulting in their declaration of war on September 3, 1939, in accordance with the defense treaties with Poland that they had signed and publicly announced . </P> <P> Germany attacked the Soviet Union in June 1941 . Hitler believed that the Soviet Union could be defeated in a fast - paced and relentless assault that capitalized on the Soviet Union's ill - prepared state, and hoped that success there would bring Britain to the negotiation table, ending the war altogether . </P> <P> Usually, the US government and the American public in general had been supportive of China, condemning the colonialist policies of the European powers and Japan in that country, and promoting a so - called Open Door Policy . Also, many Americans viewed the Japanese as an aggressive or inferior race, or both . The Nationalist Government of Chiang Kai - shek held close relations with the United States, which opposed Japan's invasion of China in 1937 that it considered an illegal violation of the sovereignty of the Republic of China, and offered the Nationalist Government diplomatic, economic, and military assistance during its war against Japan . Diplomatic friction between the US and Japan manifested itself in events like the Panay incident in 1937 and the Allison incident in 1938 . </P> <P> Reacting to Japanese pressure on French authorities of French Indochina to stop trade with China, the U.S. began restricting trade with Japan in July 1940 . The cutoff of all oil shipments in 1941 was decisive, for the U.S., Britain and the Netherlands provided almost all of Japan's oil . In September 1940, the Japanese invaded Vichy French Indochina and occupied Tonkin in order to prevent China from importing arms and fuel through French Indochina along the Sino - Vietnamese Railway, from the port of Haiphong through Hanoi to Kunming in Yunnan . This tightening of the blockade of China made a continuation of the drawn - out Battle of South Guangxi unnecessary . The agreement also allowed Japan to station troops in the rest of Indochina, though this did not happen immediately . </P>

What was one reason for the outbreak of world war 2