Answer:
Kennedy refers to the post-war period as a "hard and bitter peace" because, after the end of World War II, the Cold War began almost immediately, putting humanity back on the edge of a war, this nuclear time. Thus, the postwar period involved a coordinated effort between the different nations of the world with the aim of maintaining peace, which was not easy at all given the clear antipathy between the Western Bloc and the Eastern Bloc.
Even during this period proxy wars such as those of Vietnam, Korea, Afghanistan, etc., were developed, where America and the USSR sought to weaken the enemy through indirect confrontations.