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.
When Germany invaded Poland on 1 September 1939, triggering a declaration of war from France and the UK, it gained control of an additional two million Jews, reduced to around 1.7 – 1.8 million in the German zone when the Soviet Union invaded from the east on 17 September.