Germany with Russia allied with France conceived a war plan to rapidly defeat France before the superior resources of these two countries could be brought to bear on Germany. Because the French had heavily fortified the border, the German Schliffen Plan called for a massive stike through neutral Belgium to avoid the French fortifications along the French-German border. The Germans invaded Belgium (August 4, 1914).

Germany began the war on the Western front before it was declared, and on 1-2 August German cavalry crossed the French frontier between Luxemburg and Switzerland at three points in the direction of Longwy, Lunéville, and Belfort. But these were only feints designed to prolong the delusion that Germany would attack on the only front legitimately open to warfare and to delay the reconstruction of the French defence required to meet the real offensive.

The decision of Adolf Hitler to invade Poland was a gamble. The Wehrmacht (the German Army) was not yet at full strength and the German economy was still locked into peacetime production. As such, the invasion alarmed Hitler's generals and raised opposition to his command - and leaks of his war plans to Britain and France.