The historical context for that statement is it was made when America had a monopoly on nuclear weapons and Stalin's Soviet Union, a dictatorship of incomparable authoritarianism and delusion, was on the cusp of developing such weapons. Few would have predicted that a nuclear war would not occur once that happened. Under these circumstances, a first strike followed by occupation may have been rational. And given the complete insanity, cruelty and dehumanizing brutality of Soviet policy in the following 50 years, it's possible American occupation of the Soviet union would have saved many lives on net. See East vs West Berlin, for example. His position on first-strike was not very irrational given what was known at the time. Even in hindsight, it is hard to say either way.