Final answer:
The Reagan Doctrine changed US foreign policy by actively attempting to roll back communism worldwide and relying on supporting proxies rather than direct US military involvement. It increased defense spending but also sought to decrease direct confrontations, leading to treaties and negotiations.
Step-by-step explanation:
The Reagan Doctrine significantly altered United States foreign policy in two major ways. First, it signaled a shift from merely containing communism to actively rolling back its influence globally. This was exemplified by the U.S. providing aid to any group that was fighting against communist forces, a strategy that involved supporting insurgencies in Latin America, Asia, and Africa. The second major change was the reliance on allies and proxy groups, rather than the direct deployment of U.S. military forces. This approach aimed to avoid the heavy costs of direct military engagement experienced in Korea and Vietnam, by supporting anti-communist forces such as the Nicaraguan Contras and the Afghan Mujahedin.
Another related aspect of the Reagan Doctrine was the increase in defense spending by 35 percent, illustrating the peace through strength approach. Despite this increased spending, Reagan sought improved relations with the Soviet Union, ultimately leading to the Intermediate-Range Nuclear Forces (INF) Treaty. Additionally, the Iran-Contra Affair and direct military interventions such as the invasion of Grenada were all underpinned by the principles of the Reagan Doctrine.