What changed about the Cold War between the Soviets and the U. S. between 1970 and 1981 and what returned both countries back to the Cold War tensions?

The years between 1970 and 1981 are the years of détente, a reduction in tensions between East and West, of strategic arms controls and negotiations, of reduction of nuclear weapon stockpiles. It was a time of peaceful coexistence of capitalism and socialism, as a popular term during those years defined. President Richard Nixon and Soviet Secretary-General Leonid Brezhnev signed the SALT II treaty; Nixon also went to Beijing to make an opening in the relationships with China, and he also withdrew most troops from Vietnam after the Paris accords. This period ended with the presidency of Ronald Reagan, who launched new weapons programs and wanted to develop a space military program, putting an enormous pressure on an stagnated Soviet economy and facilitated negotiations on new disarmament treaties with the Soviet Union.