"Imperialism" is a misnomer, especially because of the confusion it can cause with Japanese expansionism in WWII (which Mishima didn't support).

Mishima was obsessed with the ideals behind the Meiji restoration, where pure, honourful samurai were to get rid of the corrupted military government of the Tokugawas to return power to the "natural" ruler of Japan, the Son of the Sun, and to purer values, including a return of the warriors to the real spirit of the Bushido which had been lost under the dictatorship of the Shoguns (a great deal of Japanese historians would say Mishima's highly idealized warrior virtually never existed, no more than his idealized vision of the rule of the Emperors during the Heian era). What Mishima wished for was a return to what he saw as the purity of the values of very ancient Japan, before the civil wars era and the Shoguns. That was an ideal behind Meiji, but not how it turned out. Mishima had a rather utopian vision of ancient Japan. He had a very poor opinion of the corrupted Japanese imperial military and the corrupted post-war governements. He thought modern Japan had lost its soul, and wished it would turn back to its lost "golden age" to find it again.