Europe’s attempts to appease Hitler failed, as Nazi Germany swallowed up Austria and Czechoslovakia by 1939. The straw that broke the camel's back, however, was Germany's invasion of Poland on September 1 of that year. Two days later, France and Great Britain declared war on Germany, and World War II had begun in earnest. 

Of course, as most people now know, the invasion of Poland was merely the preface to the Nazi blitzkrieg of most of Western Europe.The narrative is that the British stood alone in 1940 against the Nazi onslaught, defending the British Isles during the Battle of Britain and preventing a potential German invasion.

At the beginning of 1941, it was unclear whether the Allies would be able to remain in the war for much longer. British Prime Minister Winston Churchill had already immortalized the men of the Royal Air Force with one of the West’s most famous war-time quotes, but the potential of a German invasion of Britain still loomed. The Allied victory in the Battle of Britain inflicted a psychological and physical defeat on the Luftwaffe and Nazi regime at large, and as the last standing bastion of democracy in Europe, Britain would provide the toehold for the June 1944 invasion of Europe that liberated the continent. For those reasons alone, the Battle of Britain was one of the decisive turning points of history’s deadliest conflict. 

The British sought American help in North Africa, where British General Bernard Montgomery was fighting the legendary “Desert Fox”, General Erwin Rommel. At the same time, Stalin was desperate for Allied action on the European continent that could free up the pressure on the besieged Soviets. President Roosevelt had a consequential decision to make, and he eventually decided to land American forces on North Africa to assist the British against Rommel, much to Stalin’s chagrin. But the Red Army’s tenuous hold continued to cripple the Nazi war machine while buying the other Allies precious time.

With the Axis forced out of North Africa, the Allies had freed up its North African forces for an invasion of Western Europe. Moreover, with North Africa as a potential staging around for that invasion, the Germans had to prepare for the possibility of the Allies invading not only from Britain but also from North Africa. The Allies would make that decision in early 1943.

During the first half of 1944, the Americans and British began a massive buildup of men and resources in England, while the military leaders devised an enormous and complex amphibious invasion of Western Europe. In June 1944, the Allies waited for the right weather to stage the largest, most complex invasion in military history. Throughout the summer, Allied forces advanced east along a wide front, liberating vast swaths of France and Western Europe. On August 25, 1944, the Allies finally liberated Paris.