- Germany's military successes in World War 1. Germany had defeated Russia, France, and Austria-Hungary in World War 1, and its military was widely regarded as one of the strongest in the world.
- The Molotov-Ribbentrop Pact. In August 1939, Germany and the Soviet Union signed the Molotov-Ribbentrop Pact, a non-aggression pact that divided Eastern Europe between the two countries. This gave Germany a free hand to invade Poland without fear of Soviet intervention and meant that the allies would be fighting a war on two fronts if Germany invaded Poland.
- The Maginot Line. France had built a massive line of fortifications along its border with Germany, known as the Maginot Line. The French believed that the Maginot Line would be impregnable to German attack, and they were slow to respond when the Germans invaded in May 1940.
As a result of these factors, the British and French were ill-prepared for the German invasion of Poland in September 1939. The Germans quickly overran Poland, and the British and French were forced to declare war on Germany.
The British and French were able to halt the German advance in France in 1940, but they were ultimately defeated in the Battle of France. The fall of France was a major turning point in the war, and it allowed Germany to focus its attention on the Soviet Union.
The British were the only Allied power left standing after the fall of France, and they faced the prospect of a prolonged and bloody war against Germany. However, the British were able to hold out until the United States entered the war in December 1941.