he wasn't for literally declaring war, just accepting the real situation they were in: a socialist country in a capitalist world. stalin desperately tried to ally with the west against the nazis and they basically spit in his face (cus the west was always at war with the USSR despite not declaring it) so then he went to ally with the fucking nazis (and got some success). Trotsky just was admitting the truth that everyone knew, socialists and capitalists cannot cooperate. The capitalists will always do whatever they can to destroy the socialists.

Plus the fact that he realized the truth that we're living in now, that a socialist country cannot succeed while a global capitalist empire is in place. So he wanted to start weakening that empire, the same thing China has been doing for it's entire existence as a socialist state and the same thing any real leftist wants.