War Begins. In the early hours of Friday, September 1, 1939, the German armies stormed across the Polish frontierwithout a declaration of war. The Free City of Danzig (the presentday city of Gdansk, Poland, which was then an independent territory) was proclaimed a German city by Nazi agitators in the city. Two days later Great Britain and France declared war on Germany.Important dates in Europe and northern Africa: 1939-1942