Japan had made themselves a global player after the Meji Restoration and was on par with regards to their Navy with their Pacific peers like USA/UK/France/Russia. The Soviet Union had essentially moved most of their attention and focus to the European side of their country, especially with all of the nastiness going on with Stalin’s rise to power, the purges, etc.
What you get is the only westernized nation in that area that both has the might and the will to project beyond their own borders. They leveraged things like the Qing dynasty collapse to get footholds on the mainland and use their naval power to project their will onto several islands (including Taiwan). Their involvement in World War 2 was a culmination of events that predated Hitler for sure.
Add into that the Japanese way of doing things, combined with the new ideas of Nationalism, backed up by years of fighting each other and repelling foreign foes, and you get a very militaristic country that needs an outlet. Hence, they became very strong. However, a lot of that strength was propped up by American oil and gas for their Navy. One thing people don’t tend to remember about Pearl Harbor, is that it was one of something like 25 operations they started at the same time. Including seizure of the Philippines and moving into Dutch Indonesia in an attempt to secure another source of oil and gas because they were effectively cutting the cord with the USA. They also struck some serious blows with Pearl Harbor, and enjoyed quite a bit of time as the preeminent Naval power in the Pacific. The USA wouldn’t respond at Guadalcanal for over 8 months, and even that was at the far border of Imperial Japan’s claimed territories. It’s like if somebody had struck back at the US by invading Alaska or Hawaii.