On September 1, 1939, the German army under Adolf Hitler launched an invasion of Poland that triggered the start of World War II (though by 1939 Japan and China were already at war). The battle for Poland only lasted about a month before a Nazi victory. But the invasion plunged the world into a war that would continue for almost six years and claim the lives of tens of millions of people.

Today, 75 years later, Hitler is regarded as one of history's great villains. So it's easy to forget how slowly and reluctantly the worlds most powerful democracies mobilized to stop him. France and Britain did declare war on Germany two days after the invasion of Poland, but it would take them another eight months before they engaged in full-scale war with the Nazis. The United States wouldn't join the war against Hitler until December 1941, a full two years after the war began.

Why did Adolf Hitler invade Poland?

The short answer is that Adolf Hitler was a ruthless dictator with dreams of conquering all of Europe. Annexing Poland was a step in that larger plan. The Polish military wasn't powerful enough to resist him, and Hitler calculated — correctly, as it turns out — that Europe's other powers wouldn't intervene in time.

The invasion of Poland occurred almost exactly 25 years after the start of World War I in August 1914. That war ended in Germany's defeat, and in 1919 the victorious allies carved up territory that had been part of Germany, Austria-Hungary (Germany's defeated ally), and Russia (which had fallen to the Bolsheviks) into an array of new countries.

One of these new countries was Poland, which before 1919 had last existed as an independent nation in 1795. Another was Czechoslovakia — its awkward name reflects the Allies' decision to combine areas dominated by two different ethnic groups, Czechs and Slovaks, into a single nation.

Hitler was contemptuous of these new nations, which he regarded as artificial creations of the Allies. There were significant German populations in both countries, and Hitler used trumped-up concern for their welfare as a pretext to demand territorial concessions.

In the infamous 1938 Munich Agreement, British Prime Minister Neville Chamberlain agreed to Hitler's annexation of the Sudetenland, portions of of Czechoslovakia with ethnic-German majorities (Czechoslovakia itself was excluded from the negotiations). Chamberlain claimed that the deal had averted another massive European war, but it only delayed the conflict while making Hitler more powerful when the war finally came.

Chamberlain's accommodating stance in the 1938 negotiations convinced Hitler that the British and French wouldn't seriously resist further annexations to his east. And in any event, Hitler calculated — correctly as it turned out — that he could conquer Poland before the Allies could do anything to stop him.

How did the Soviet Union react to the invasion of Poland?

You might have expected a German invasion of Poland to set off alarm bells in Moscow. Germany and Russia were historic enemies, having fought each other during World War I. Moreover, Hitler and Soviet leader Joseph Stalin were theoretically at opposite ends of the political spectrum — the Communists and Nazis had viewed each other warily throughout the 1930s.

But the Allies' handling of the Sudetenland crisis spooked Stalin. He feared that Hitler would seek to annex portions of the Soviet Union next. He thought that the Western Powers — who had no love for either Hitler or Stalin — would be happy to leave the Communists to face the Nazis alone.

So in August 1939, these historic enemies signed a non-aggression pact. The deal shocked the Allies, who had counted on the Soviet threat checking Hitler's territorial ambitious. What London and Paris didn't know was that the deal included secret provisions outlining how the two powers would divide up the smaller nations that lay between them — including Poland.

So when German troops crossed the border into Poland, Stalin not only didn't object, he began making plans for his own invasion of Poland from the East.

How effective was Polish resistance?

Not very. Poland was determined to resist Germany's invasion, and on paper it had a decent shot at doing so. Poland had 1.3 million troops against Germany's 1.5 million, and Polish troops were highly motivated.

But the Polish military was no match for Hitler's war machine. While Poland and Germany deployed similar numbers of men, Germany's troops were much better supplied. According to historian Max Hastings, Germany had 3600 armored vehicles against 750 in Poland. Germany had twice as many airplanes as Poland did — and its planes were more advanced.

So Poland found itself overmatched. And because the German army in 1939 was a lot more mechanized than it had been in previous wars, the Germans were able to make progress extremely quickly. A little over a week after the start of combat, German troops had reached the outskirts of the Polish capital, Warsaw. It fell on September 29.

The Polish situation became even grimmer on September 17, when Russian troops began pouring across the border from the East. The Polish army had already been at a disadvantage, but when the Soviets attacked the Polish situation became hopeless. German and Russian troops secured full control over Poland by October 6, 1939.

How did the Allies respond to the invasion?

The British and the French had both promised to declare war on Germany in the event of an invasion of Poland. But after Munich, Hitler doubted that Chamberlain had the stomach to go to war in defense of Poland. He was wrong. Chamberlain was furious that Hitler had broken his Munich promise to seek no more territorial gains beyond Czechoslovakia. On September 3, both France and Britain kept their promise and declared war on Germany.

But while both countries could declare war, neither was really prepared to wage it. They had begun mobilization a few months earlier to prepare for possible German aggression, but the two countries still felt a few more months of military buildup would put them in a stronger position to fight the Nazis.

While they didn't expect the Polish to beat the Germans, they expected Poland's million-man army to put up a stronger resistance, bogging down the German troops and giving the Allies time to make war plans.

Obviously, that calculation proved to be a mistake, both because the Germans were more powerful than expected and because the Allies hadn't counted on the Soviets invading Poland as well. So Poland fell before the Allies could launch more than token attacks on the Nazis.

What happened next?

Once Hitler and Stalin had consolidated their combined control over Poland, the Western Allies felt even less pressure to attack Germany quickly. Warsaw had already fallen, so there was no Polish government to save from defeat. The Allies controlled the seas and believed they had time on their side.

The Allies were also haunted by memories of World War I, in which millions of lives had been lost to no obvious purpose. They hesitated to launch into a second continent-wide war with little apparent strategic purpose.

In this sense, the opening months of World War II were the complete opposite of World War I. In the earlier war, hostilities on the Western Front began almost immediately after war was declared in early August. Indeed, much of the action in the First World War occurred in the first six weeks, with German troops nearly reaching Paris before getting bogged down into trench warfare.

In contrast, there was an eight-month gap between France's declaration of war against Germany in September 1939 and the beginning of full-scale war between Germany and France in May 1940. During this period, the continent was technically at war, but not much actual fighting was happening. Some in Britain and France still hoped that a solution to the crisis could be found without the loss of millions of lives. Only after German tanks began to stream into France on May 10, 1940 did the Allies fully accept that stopping Hitler would require another full-scale world war.

How did the Polish cope with occupation?

According to historian Max Hastings, "Poland became the only nation occupied by Hitler in which there was no collaboration between the conquerors and the conquered." Historians estimate that about 5.5 million Polish people died under the Nazi occupation of their country, half of whom were Polish Jews. Another 150,000 died under Soviet rule.

Nazi propaganda portrayed the Poles as having oppressed ethnic Germans in Poland, and they used this as a pretext for subjecting the Polish people to ethnic cleansing and slavery. The Poles responded by organizing one of the largest resistance movements in Nazi-occupied territory. Polish nationalists sabotaged German production facilities and disrupted German supply lines. Polish nationalists organized the unsuccessful Warsaw Uprising to throw off Nazi rule in 1944.

Unfortunately, the defeat of the Nazis in 1945 did not bring about Polish freedom. Poland was "liberated" by the Soviet Union, which installed a repressive communist regime there. Poland would be trapped behind the Iron Curtain until the Polish people finally threw off Communist rule in the 1980s.

Update: I replaced a sentence about Polish non-cooperation with the Nazis with a quote from Max Hastings.