The existence of the Axis alliance was a major cause of World War II. Although Germany became more and more militaristic throughout the 1930s, Hitler knew that he could not start his conquest of Europe without forming alliances. By joining up with Italy, another fascist nation, Hitler was able to ensure that Germany would not have to fight alone on the southern front. Hitler also knew that it would be disastrous to fight an all-out two-front war right out of the gate. That is why he also formed the non-aggression Molotov-Ribbentrop Pact with the Soviet Union in August 1939.
When Germany invaded Poland in September 1939, the French and British put into action their previous alliance by which they had pledged to protect Polish independence. As a result, they quickly declared war on Germany. This is often considered the beginning of World War II.
Since Stalin was confident in the pact he had signed with Hitler, he ordered the Soviet army to invade eastern Poland. Germany and the Soviet Union set about setting up their spheres of influence in Eastern Europe. Germany quickly defeated France at the same time.
After Germany's defeat of France, Italy felt confident enough to enter the war. Hitler had hoped that the Italians would be able to secure Europe's southern front. When the Italian military proved incapable of victory in the Balkans and North Africa, Germany was forced to send its military there to conquer those territories.
Shortly after Japan attacked Pearl Harbor on December 7, 1941, Germany declared war on the United States. This may have been an error on Hitler's part. By siding with its fellow Axis power, Germany drew the United States into the conflict in Europe. It is plausible the United States might have otherwise focused exclusively on fighting Japan.