World War II did not begin with a single act of aggression; it began with two decades of unresolved grievances, failed diplomacy, and a series of calculated gambles that Western democracies kept declining to stop. When German tanks crossed into Poland on September 1, 1939, it was the culmination of a chain of decisions stretching back to the peace that ended World War I — and one secret deal, signed just days earlier, that made the invasion possible.

A Punishing Peace Set the Stage

The Treaty of Versailles, signed in 1919, forced Germany to accept territorial losses, drastic military restrictions, and heavy reparations payments as the price of losing World War I. The treaty was intended to prevent future German aggression, but instead it fueled deep, lasting resentment across German society, which viewed the terms as a national humiliation. Combined with the economic devastation of the Great Depression in the early 1930s, that resentment created fertile ground for extremist politics.

Hitler’s Rise and Early Violations

Adolf Hitler became German chancellor in January 1933 and rapidly consolidated the Nazi Party’s hold on a one-party totalitarian state, openly vowing to reverse the Treaty of Versailles. He began rearming Germany in defiance of the treaty and, in March 1936, sent troops to remilitarize the Rhineland, a region that had been demilitarized under the treaty’s terms. Around the same time, fascist Italy under Benito Mussolini invaded Abyssinia (modern Ethiopia) in October 1935. Both moves violated international agreements, and both went unpunished, as the League of Nations proved unable to enforce consequences on major powers.

Appeasement and the Munich Agreement

Eager to avoid a repeat of World War I’s carnage, Britain and France pursued a policy of appeasement — conceding to German demands in hopes of preserving peace. That approach reached its peak in September 1938, when British Prime Minister Neville Chamberlain and French leaders signed the Munich Agreement, permitting Hitler to annex the Sudetenland, a German-speaking region of Czechoslovakia, without Czechoslovak consent. Chamberlain declared it had secured “peace for our time.” Instead, in March 1939, Hitler seized the rest of Czechoslovakia outright, convincing Britain and France that appeasement had failed and that his ambitions had no natural limit.

The Secret Pact That Cleared the Way

With Poland now the evident next target, Britain and France pledged to defend Polish independence — but Hitler still faced the risk of a two-front war against both the Western powers and the Soviet Union. He solved that problem on August 23, 1939, when German Foreign Minister Joachim von Ribbentrop and Soviet Foreign Minister Vyacheslav Molotov signed the Molotov-Ribbentrop Pact in Moscow, a nonaggression treaty between Nazi Germany and the USSR. A secret protocol attached to the pact divided Eastern Europe into spheres of influence, with the Soviet Union set to receive eastern Poland, Lithuania, Latvia, and Estonia once Germany moved on the rest.

September 1, 1939: The War Begins

With his eastern flank secured, Hitler ordered the invasion of Poland on September 1, 1939. Britain and France, honoring their guarantee to Poland, declared war on Germany on September 3, 1939, formally beginning World War II in Europe. The Soviet Union invaded Poland from the east on September 17 under the pact’s secret terms. That German-Soviet alignment would not last: on June 22, 1941, Hitler broke the pact entirely and launched Operation Barbarossa, a massive invasion of the Soviet Union, drawing the USSR into the war on the side of the Allies alongside Britain and, from December 1941, the United States.