The Invasion of Poland Wasn’t Hitler’s First Aggression. Here’s Why That Move Marked the Beginning of WWII

Roughly 1.5 million German soldiers, 2,000 airplanes and 2,500 tanks crossed the Polish border on Sept. 1, 1939

After roughly 1.5 million German soldiers, more than 2,000 airplanes and more than 2,500 tanks crossed the Polish border on Sept. 1, 1939, the British gave Nazi dictator Adolf Hitler an ultimatum: pull out of Poland, or else. Hitler ignored the demand, and two days later, on Sept. 3, 1939, Britain and France declared war. Thus began World War II, and this weekend Vice President Mike Pence will travel to Poland to mark the anniversary of that event.

But the invasion of Poland wasn’t the first time German forces had been put to work for Hitler’s goal of European domination. Previously, however, the other European powers had pursued a strategy of appeasement, giving Hitler what they deemed reasonable concessions, in order to avoid all-out war. That strategy reached its apex when the three parties signed the Munich Agreement on Sept. 30, 1938, giving Hitler the German-speaking part of Czechoslovakia, known as the Sudetenland, on the condition that he would not invade any more territory. But six months later, in March of 1939, Hitler violated the Munich Agreement by absorbing all of Czechoslovakia.

The war didn’t begin then. Rather, it took another half a year.

Rumors started swirling that Hitler was eyeing Poland next. With French support, Britain promised on March 31, 1939, that if Germany made aggressive moves toward Poland, they would come to Poland’s defense. By the time that happened, not only had Hitler broken yet another promise, something else had shifted too. “When Hitler invades Poland in ’39 there is no political support any longer for appeasement,” explains Rob Citino, Senior Historian at The National WWII Museum.

Though France urged Britain to wait, says Tim Bouverie, author of Appeasement: Chamberlain, Hitler, Churchill, and the Road to War, many British politicians feared the implications of not keeping the promise to Poland, and they were done giving Hitler the benefit of the doubt.

“Hitler had proven, by tearing up the Munich agreement and invading Czechoslovakia in March of that year, that he could not be trusted and that he had to be stopped,” Bouverie says. By falsely claiming that he only wanted to fix damage done to Germany from World War I and restore German lands to German people, Hitler had previously been able to convince his counterparts—already wary of war—to hold off. “Both of these claims are proven as lies when he invades Czechoslovakia in March 1939 and the British government realizes that he is intent upon wider European conquest—possibly domination.”

Prime Minister Neville Chamberlain laid out the argument for ending the appeasement strategy in a Sept. 4 radio address aimed at the German people: “He gave his word that he would respect the Locarno Treaty; he broke it. He gave his word that he neither wished nor intended to annex Austria; he broke it. He declared that he would not incorporate the Czechs in the Reich; he did so. He gave his word after Munich that he had no further territorial demands in Europe; he broke it. He has sworn for years that he was the mortal enemy of Bolshevism; he is now its ally.”

Hitler’s propaganda endorsed the theory of Lebensraum (often translated as “living space”), his idea that the Germany needed more room. Citino points out that Poland was geographically the logical next step after Czechoslovakia, in terms of the application of that theory. In addition, the dictator believed that the Polish population was racially inferior to Germans, and thus would be easily overrun and enslaved. (On Sept. 17, the Soviet Union also invaded Poland, in accord with a non-aggression agreement Hitler and Stalin had come to that summer; that agreement would end on June 22, 1941, when the Nazis invaded Soviet territory.)

“It seems Hitler can no longer be appeased [in 1939], but attempting to appease him was wrong all along,” Citino says. “He would just continue to make demands and threaten his neighbors ad infinitum.”

Here’s how TIME described the Nazi invasion of Poland in its Sept. 11, 1939, issue:

World War II began last week at 5:20 a. m. (Polish time) Friday, September 1, when a German bombing plane dropped a projectile on Puck, fishing village and air base in the armpit of the Hel Peninsula. At 5:45 a. m. the German training ship Schleswig-Holstein lying off Danzig fired what was believed to be the first shell: a direct hit on the Polish underground ammunition dump at Westerplatte. It was a grey day, with gentle rain.

In the War’s first five days, hundreds of Nazi bombing planes dumped ton after ton of explosive on every city of any importance the length & breadth of Poland. They aimed at air bases, fortifications, bridges, railroad lines and stations, but in the process they killed upward of 1,500 noncombatants. The Nazi ships were mostly big Heinkels, unaccompanied by pursuit escorts. Germany admitted losing 21 planes to Polish counterattack by pursuits and antiaircraft. They claimed to have massacred more than half of a 47-plane Polish squadron which tried to bomb Berlin.

Out of a welter of sketchy bulletins, counter-claims and unpronounceable names flowing from Poland, the broad outlines of Germany’s assault began to take shape. Recapture of what was Germany in 1914 was the first objective: Danzig, the Corridor, and a hump of Upper Silesia. It is believed that Adolf Hitler, if allowed to take and keep this much, might have checked his juggernaut at these lines for the time being. When Britain & France insisted that he withdraw entirely from Polish soil or consider himself at war with them, he determined on the complete shattering and subjugation of Poland…

Heroes this week were a handful of Polish soldiers left in charge of the Westerplatte munitions dump. Under steady bombing and shell fire, they held out as a suicide squad in the thick-walled fortress, replying from its depths with machine gun fire, resolved to blow up the dump and themselves with it before surrendering.

Another small band of Poles took and held the Danzig post office until artillery was drawn up to blow away the building’s face, gasoline poured on from above and set afire.

On “Black Sunday”—the day Britain and France declared War—the President of the United States Franklin D. Roosevelt announced, “This nation will remain a neutral nation, but I cannot ask that every American remain neutral in thought as well. Even a neutral has a right to take account of facts. Even a neutral cannot be asked to close his mind or his conscience.”

As TIME pointed out, the sentence was “the most striking sentence in the broadcast” because of the contrast with President Woodrow Wilson’s 1914 edict that Americans must remain “impartial in thought as well as action” in the early years of World War I. The Roosevelt version suggested to the magazine that the president might be priming Americans to get ready to take up arms—and after the attack on Pearl Harbor in 1941, they did.

The lead-up to World War II, Bouverie says, was about “what bad people are able to do when they think that the good people aren’t prepared to fight.” The fighting, however, would come in the end.