On September 1st, 1939, the German army, the Wehrmacht, invaded Poland, prompting, finally, European intervention against Adolph Hitler's government and setting into motion World War II.
The invasion of September 1939, however, was in a result, not the beginning, of German aggression. Not long after taking power in 1933, Adolph Hitler began violating various terms of the Versailles settlement. In just a few years, the Germans rearmed, remilitarized the Rhineland, occupied Austria as part of the anschluss, took over the Czech Sudeten region, and then took all of Czechoslovakia.
While Hitler's aggression raged, the west did almost nothing. The British and French, still reeling from their horrific losses in World War I, alienated by the sense of disillusion that came with the aftermath of that war, and not really shocked by the Nazi treatment of Jews, which was not atypical of the European ruling class, followed a policy of appeasement. In fact, Europe's elite was more concerned about the Soviet Union and communism, so saw Hitler as a welcome barrier to "Stalinist expansion."
In the United States, also disillusioned by a war which saw, as Fitzgerald put it, "all gods dead, all wars fought, all faiths in man shaken" and incensed by the findings of the Nye Committee about War Profiteering, isolationism and isolationist legislation made it clear that the administration of Franklin Roosevelt would not act against Nazi expansion (nor, for that matter, against Italian or Japanese aggression).
Indeed, when the west had a chance to confront the Fascists, in Spain in 1936-39, they sat it out. During a civil war there, with the Republican government under attack from Falangist forces led by Generalissimo Francisco Franco [still seriously dead] and supported by the Germans, only Stalin offered support, while the Nazis used Spain as a laboratory for weapons and doctrine.
By 1938, appeasement reached its denouement, as the western nations met at Munich and accepted Hitler's conquest of Czechoslovakia [the Czechs weren't there]. Left with no allies, the Soviet Union, which had been sounding the alarums about Hitler since 1933, had to make an alliance with the Nazis to protect their own territories (an agreement which Hitler broke in June 1941 when he attacked the USSR).
By 1939, then, the west had failed to confront Fascist aggression many times and Germany had been able to rebuild its economy and rearm without any serious barriers. So it had a window of opportunity, where its industrial and military capacity was adequate for war, and it took that opportunity with the invasion of Poland on 1 September, justified because so many ethnic Germans lived in the Danzig Corridor.
The invasion itself was short-lived and the Polish were decimated
by the German blitzkrieg, or "lightning war." In just a brief time, Germany
had assumed control over Poland. The British and French declared war against
Germany, but did not intervene, as they were not adequately mobilized and
they perceived Poland as a lost cause.
After several months of so-called Phony War, the Germans struck
again, conquering France in a dramatic and lighting campaign in the Spring
of 1940. By that time, it was clear that a world war was looming, in large
measure because the west had failed to confront Hitler prior to September
1st, 1939.
Further Reading:
A.J.P. Taylor, The Origins of the Second World War
William Appleman Williams, The Tragedy of American Diplomacy
Michael J. Carley, 1939: The Alliance that Never Was and the Coming
of World War II
D.C. Watt, How War Came
David Reynolds, From Munich to Pearl Harbor
Arnold Offner, America and the Origins of World War II
Lloyd Gardner, Economic Aspects of New Deal Diplomacy