In World War II, Japan met American territory in the Pacific Ocean. The Japanese believe that the Pacific Ocean should be unique to Japan and cannot be shared with other countries. Coupled with the victory of World War I and Germany, a powerful ally of World War II, Japan decided to give it a try. If it wins, Japan will enter the ranks of world powers. If you lose, you will lose decades of national luck. Finally, Japan failed and became a defeated country.
After World War II, the Cold War broke out, and the shrewd Japanese realized that it was impossible to make a comeback on their own. The United States needs a forward base in Asia to fight against the Soviet Union. If it stands on the side of the United States, it will undoubtedly get economic and military assistance from the United States to get rid of backwardness. Fortunately, Japan did not stand wrong this time.
Now, with the economic support of the United States, Japan has surpassed the former victorious country in just a few decades after the war, firmly attached to this powerful ally of the United States, and its national security has been guaranteed (so it has only recently dared to compete with Russia and China for islands).
The Japanese have always been shrewd businessmen, and they know the wind very well. Now, because the Americans are strong, Japan needs the shelter of the United States, and the United States needs Japan, a military base that is advancing in Asia. Both sides have their own needs, forming the status of superpower and vassal state, but the Japanese have never stopped playing with their own hearts. Once the United States does not meet its own interests and has a stronger ally, it will also kick the United States away.