On 1 September 1939, Germany invaded western Poland under the pretext of having been denied claims to the Free City of Danzig and the right to extraterritorial roads across the Polish Corridor, which Germany had ceded under the Versailles Treaty.
In 1939 Kennedy toured Europe, the Soviet Union, the Balkans, and the Middle East in preparation for his Harvard senior honors thesis. He then went to Czechoslovakia and Germany before returning to London on September 1, 1939, the day that Germany invaded Polandto mark the beginning of World War II.
The United Kingdom responded with an ultimatum to Germany to cease military operations, and on 3 September, after the ultimatum was ignored, France and Britain, along with their empires, declared war on Germany. The alliance provided no direct military support to Poland, outside of a cautious French probe into the Saarland.
On 4 September 1939, the day after the UK declared war on Germany, Turing reported to Bletchley Park, the wartime station of GC&CS. Specifying the bombe was the first of five major cryptanalytical advances that Turing made during the war. The others were: deducing the indicator procedure used by the German navy; developing a statistical procedure for making much more efficient use of the bombes dubbed Banburismus; developing a procedure for working out the cam settings of the wheels of the Lorenz (Tunny) dubbed Turingery and, towards the end of the war, the development of a portable secure voice scrambler at Hanslope Park that was codenamed, Delilah.
The Polish counter offensive to the west halted the German advance for several days, but it was outflanked and encircled by the Wehrmacht. The Battle of the Bzura fought between 9 and 19 September 1939, It began as a Polish counter-offensive, but ended with German victory, German forces took all of western Poland.
In November 1939, the United States was taking measures to assist China and the Western Allies, and amended the Neutrality Act to allow "cash and carry" purchases by the Allies. Cash and carry was a policy by US President Franklin Delano Roosevelt announced at a joint session of the United States Congress on September 21, 1939, subsequent to the outbreak of war in Europe. It replaced the Neutrality Acts of 1937, by which belligerents could purchase only nonmilitary goods from the United States as long as the recipients paid immediately in cash and assumed all risk in transportation using their own ships.