after the agreement that ended the cuban missile crisis, relations between both sides eased up. several treaties, designed to reduce the number of nuclear weapons, were signed. in 1964, the us under president lyndon johnson invaded north vietnam, which resulted in a humiliating defeat for the americans and south vietnam in 1975. during this period of détente, the united states began building a good relationship with the people's republic of china, which had once been an ally of the soviet union.