As the other answer said, World War II began when Germany invaded Poland in September 1939.
Up until this time, powerful countries in Europe, in particular, England and France, had followed a policy of appeasement toward Germany. This meant that they gave into Hitler's demands for more territory for the German Reich. They accepted Hitler's argument that these lands were populated with Germans who were rightly part of the larger German state.
England and France had many reasons for wanting to avoid another war. The First World War had resulted in very high casualty rates, and it was a living memory in the minds of almost all adults alive in the 1930s. Governments knew there would be very little popular support for another war unless people thought it was absolutely necessary. Second, England and France had exhausted their finances fighting World War I. They feared they lacked sufficient resources to win another war, which was why England was so keen on a strong alliance with the United States. Finally, European states were still reeling from the Russian Revolution, another shocking event within living memory. To many, especially in the elite groups that ran the various governments, a strong Germany was a buffer zone between them and feared Soviet aggression.
Finally, however, Hitler, who wanted a war, got his wish, and the nations that had appeased him felt compelled to fight him.