The major consequence of imperialism for Japan was World War II. This was, of course, a disastrous consequence. In the longer term, another consequence has been the fact that many of Japan’s neighbors have a very negative attitude toward the country.
Japan began to be an imperial power in the early 1900s. The country was very eager to become powerful and to be seen as the equal of the European countries. Japan then went on to take more and more of an empire in East Asia. This led Japan to, eventually, take Manchuria and even invade China proper. The Japanese then took over French Indochina after World War II had broken out in Europe. This led America to be very angry with Japan, thus causing Japan to attack Pearl Harbor.
The way that Japan treated many East Asian countries during its imperial period has caused many in those countries to dislike Japan strongly. This has helped to cause tensions between Japan and, for example, China and South Korea.
Thus, Japan’s imperial period ended up hurting the country badly in many ways that have continued to be felt today.