Hitler marched his troops into the Rhine, the Sudetenland, the rest of Czechoslovakia, and Austria over the protests of Britain, France, and the US. On Sept. 1, 1939, Nazi Germany invaded Poland, thereby starting WW2 in Europe. The Poles fought valiantly against Hitler's forces; when they took refuge behind the Vistula River, and it seemed they could hold on until the arrival of the British and French, Stalin entered the war and invaded Poland from the east.