Final answer:
During Ronald Reagan's presidency, the Cold War saw a shift from increased defense spending and hardline rhetoric to groundbreaking diplomatic efforts with the Soviet Union, leading to the signing of the INF Treaty, and ultimately contributing to the end of the Cold War by 1991.
Step-by-step explanation:
The Cold War dynamics underwent significant changes during Ronald Reagan's presidency in the 1980s. Initially taking a hardline approach, Reagan increased defense spending, referring to the Soviet Union as an "evil empire," and supported insurgencies in countries like Central America. However, in the latter part of his presidency, Reagan's stance softened as he engaged in diplomacy with Soviet leader Mikhail Gorbachev, which included a shift in Soviet policies towards glasnost and perestroika, the signing of the Intermediate-Range Nuclear Forces (INF) Treaty, and eventually a significant reduction in nuclear arms. The Soviet Union's engagement in a costly war in Afghanistan, along with economic struggles, catalyzed its collapse, and by 1991 the Cold War effectively came to an end.