Short answers, pulled from the story.
The Soviet invasion of Afghanistan in December 1979 ended the era of détente and triggered a cascade of retaliatory responses from the United States, including a grain embargo, a boycott of the 1980 Moscow Olympics, and weapons sales to Afghan rebels. The elections of Margaret Thatcher in 1979 and Ronald Reagan in 1980 brought leaders committed to challenging Soviet power rather than managing coexistence.
Able Archer 83, a NATO exercise conducted in November 1983, simulated a full-scale nuclear release for the first time. Soviet military intelligence could not determine whether the exercise was a cover for an actual first strike. Military units in East Germany and Poland were placed on alert, MiG-23s were stationed near the border, and SS-20 and SS-19 missiles were moved to readiness. Many historians regard it as the closest the world came to nuclear war since the Cuban Missile Crisis of 1962.
Operation RYAN, an acronym for the Russian phrase meaning "nuclear missile attack," was the largest Soviet intelligence-gathering campaign ever launched, authorized in 1981 by KGB Chairman Yuri Andropov and General Secretary Leonid Brezhnev. Its purpose was to detect the earliest signs of an American nuclear first strike. The KGB doubled its effort under RYAN in February 1983 as tensions continued to rise.
Korean Air Lines Flight 007 departed Anchorage, Alaska on the 1st of September 1983 bound for Seoul, South Korea, but strayed over the Soviet Kamchatka Peninsula. Soviet fighters shot it down with air-to-air missiles. All 269 passengers died, including U.S. Congressman Larry McDonald of Georgia. The shootdown significantly intensified the already high political tensions of 1983.
Leonid Brezhnev led the Soviet Union until his death in 1982 after 18 years in power. Yuri Andropov succeeded him but served only 15 months before dying on the 9th of February 1984. Konstantin Chernenko then led for 13 months from February 1984 to March 1985. Mikhail Gorbachev, Andropov's preferred successor, took power in 1985 and began reducing Cold War tensions.
The Reagan Doctrine replaced détente with a policy of rollback, actively seeking to dissolve Soviet influence in Soviet-bloc countries rather than containing it. Reagan's administration pursued an aggressive military buildup, channeled approximately $750 million a year to Afghan rebels fighting Soviet forces, pushed Western European governments to host American Pershing II missiles, and announced the Strategic Defense Initiative, a proposed space-based missile defense system.