During World War I, Japan had been an ally of Britain, France, Italy, Russia and the U.S. After WWI, America decided to gain territories in the Pacific, and it formed the Pacific Defense Triangle. Japan also wanted to expand and it began a rivalry with the U.S. over the Pacific.
At the same time, Japan began expanding into Asia, particularly into China—a now weak country due to their revolutions and civil wars. In 1931 the Mukden Incident occurred and started sporadic warfare between Japan and China, which lasted until 1937. That same year (1937) the Marco Polo bridge incident came about. This officially started World War II in the East. From then until 1945 there was continuous warfare between China and Japan.
Adolph Hitler and his Nazis were another key factor in bringing forth World War II. Hitler was born in Austria-Hungary. His parents both died while he was still young. He made his living off of an inheritance and also painting post cards. Throughout his adolescence Hitler was involved in anti-Semitic movements. This is key because when he came into power Hitler was set upon practically wiping out the Jewish population in Germany and elsewhere. Hitler also fought in WWI and became a corporal.
After the war he was assigned to infiltrate a group called the DAP. He gained control of this group, made it socialistic and renamed it the NSDAP. This group was the origin of the Nazi party. During the Great Depression of 1930 the Germans began to be more sympathetic towards the communist parties, including the Nazis. In 1933, the same year Franklin D. Roosevelt became President, Hitler gained power in the German government and took over.
Following this Hitler did a few things which set him against the Allies (Britain, France, U.S.). In the Treaty of Versailles (of the Paris Peace Conference) it had been decided that Germany had to disarm and have only a minimal army. Hitler went against both these decisions and rearmed in 1934 and remilitarized the Rhineland in 1936. Two years later, Germany unified with Austria, which had been against another treaty—the Treaty of St. Germain.
Lastly there was the Polish Crisis, also involving Hitler and Germany. The Germans were persecuted in Poland and Hitler was likewise putting a lot of pressure on the Polish. In the Non-Aggression the Germans made an agreement with the Soviet Union to divide Poland up. And, in 1939, Hitler invaded Poland. This began World War II in the West.
Therefore, what began the war in the West was the Polish Crisis and Hitler’s invasion. What began the war in the East was the Japanese expansion and also it’s rivalry with the U.S., which culminated in the attack on Pearl Harbor.
Note: Hitler moved to Germany early in his career.