In my current Hearts of Iron III game, Germany began mobilizing on June 1, 1939, months after the French and Polish. In the early morning of July 1, the demands for Danzig and the Polish Corridor unmet, the Germans Invaded Poland on a paper-thin pretext. Britain, France and their minor allies declared war even as a massive force rolled across the eastern borders and tore through the Polish defenses. With heavy air support, German troops put enormous pressure on the Polish lines while three armored spearheads thrust deep into Poland, cutting off the retreat to Warsaw. Within a week both Danzig and the capital had fallen, and Lodz and Krakow followed. With most of the country’s strategic centers in German hands the Poles had no choice but to surrender on July 10, their nation annexed.
Abiding by the terms of January’s Molotov-Ribbentrop Pact, however, Germany withdrew from eastern Poland and the Soviets began to move in. Forward German infantry divisions immediately began digging in on the new border as garrison units occupied Danzig and Memel and the majority of the remaining forces began to entrain for the western front. In the West, the Netherlands and Belgium remain neutral, although both began to mobilize during the Polish campaign. Germany’s Italian allies upheld their end of the Pact of Steel by attacking along their border with France. The French had the better of these engagements, but no serious territorial gains occurred, and significant French forces remain tied down on this front.
By the end of July forces had begun to gather behind the lines in the west. Along the Maginot-Siegfreid Line, all is yet quiet.