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.

Alliances were quite important in the buildup leading to World War II. Germany and Italy formed an alliance which would soon be joined by Japan in order to form the Axis powers in 1940. Hitler sought to secure his southern flank by incorporating Italy into the group. Japan sought the Asian colonies of Britain and France when it joined the Axis powers. While this union would not last, Germany signed a temporary nonaggression pact with the Soviet Union in 1939. While this was not a true alliance in that the Soviet Union was not expected to help fight on Germany's behalf and vice versa, it is highly important, and one could argue that the war would not have started without it. When Germany invaded Poland on September 1, the Soviet Union invaded eastern Poland and the Baltic republics at the same time. Hitler knew the mistake of creating a two-front war, and this is why he wanted to ensure Soviet neutrality in the East. This alliance would not last as Hitler sought to create an eastern empire in 1941.

Britain and France joined forces to defeat Hitler after the invasion of Poland. After France was overrun in June 1940, Britain relied more and more on commercial contacts inside the United States in order to keep its war effort and economy afloat. To combat the German U-boat, Roosevelt authorized American destroyers to attack German U-boats if attacked. This would soon be modified to shooting U-boats on sight, thus making the United States a close ally of Britain before December 7, 1941. When the Soviet Union was invaded in summer 1941, it joined an alliance with Britain. Britain strongly desired to get the Soviet army on its side, as it would wear away the German force through attrition. The attack on Pearl Harbor put the United States firmly in the camp of the Allies. The war now pitted Germany, Italy, and Japan against the forces of Britain, the Soviet Union, and the United States.

The creation of alliances was a factor in the outbreak of World War II, just like it was a factor in the outbreak of World War I. In World War II, there were two major alliances. The Axis Powers consisted of Germany, Japan, and Italy. The Allied Powers consisted of the main countries of France and Great Britain. Later, the United States and the Soviet Union were members of the Allies.

The creation of alliances allowed member nations to rely on each other for support, for aid, and for resources. This was very important to the Axis Powers since they were intent on taking aggressive actions against other countries. Without support from the other alliance members, they may have been less willing to take these actions on their own. This could have left them to have to fight alone if a war broke out with other countries. For example, Germany knew it had the support of Italy and eventually Japan as Germany began to break the Versailles Treaty, build up its military, and annex other nations or lands.

The Allied alliance was important also since it allowed Great Britain and France to work together to deal with the aggression of Germany, Japan, and Italy. When the war broke out, the Allies were also able to try to coordinate their efforts to stop the Axis Powers.

Alliances were a factor in the outbreak of World War II and also during World War II.