Following the First World War, both France and Poland had reasons to fear future German military aggression. Since Prussia united the fractured German states under its leadership in the 1860s, German leaders had used military action against their neighbours to east and west both as routes to territorial aggrandisement and a way to keep Germany united. Germany was a nation with a reputation for belligerence, whose troops had marched through both countries in the First World War.
To counter this German belligerence, the French and the Polish governments agreed to a military treaty in 1921, binding them to support each other in any war against Germany. It was on the back of this treaty that, two days after the German invasion of Poland on 1 September 1939, France declared war on Germany.
At the time, the declaration of war was a largely symbolic act. Like Britain, which had declared war on the same day, France was too far from Poland to offer real aid in driving back the invaders.
But one possibility offered itself. An invasion of western Germany by French troops might draw soldiers away from the attack on Poland. Failing that, it would at least give France a head start in the war that must inevitably come her way.
