Japan took a number of steps that eventually led to the beginning of the part of World War II that was fought in the Pacific Theater. These steps culminated in the attack on Pearl Harbor that took place on December 7, 1941.
Perhaps the first step Japan took was to participate in WWI. When it did, it took Germany’s possessions in Micronesia. This was a step towards taking an empire for itself. Japan next turned towards China. In 1931, that Japanese Army staged an incident that allowed it to take control of the region of Manchuria in Northeastern China. Japan eventually invaded the rest of China. This expansionism by Japan worried the US. The next step that Japan took that led to the war was to take over the French colonies in Indochina. The Japanese were allied to the Germans, though they were not fighting anyone before December 7, 1941. As German allies, they took over France’s colonies in Indochina after France fell to the Nazis. This scared the US even more. Finally, Japan attacked Pearl Harbor. This led directly to the start of WWII in the Pacific.