U.S. foreign policy from about 1945 to 1989 revolved round fighting the Soviet Union without actually fighting the Soviet Union. There are two reasons for this. First, both Napoleon and Hitler proved that invading Russia is only slightly more productive than hitting yourself on the head with a hammer. The second is that the Soviet Union got nuclear weapons in 1949 and any war would be the apocalypse.

If the Marshall Plan was the opening salvo against the Soviet Union, President Reagan presided over the end. Some will say that he singlehandedly won the Cold War, but it's more accurate to say he built on what others did.

Under Reagan, the plan was spending. Essentially, the US kicked military spending into high gear, producing tons of both conventional and nuclear weapons. The Soviet Union had to keep up, or they could be annihilated. Whoever could spend more would win. That was the U.S. The fallout of this, unfortunately, was a ton of nuclear weapons in a state whose government was falling down around its ears.

But, hey, no more Soviet Union.

The main significance of the Reagan Doctrine as opposed to the old model as this was a shift from the accommodation of Bohlen or the containment of some other thinkers. Reagan was all about rollback. He wanted places that were either communist or socialist (he didn't always differentiate, even though there's a vast difference), to not be.

This created one of the largest features of the Reagan Doctrine: the support of right wing guerrilla armies around the globe who were either fighting the Soviets or any kind of communist, socialist, or left wing government. This included the Mujahideen in Afghanistan, the contras in Nicaragua, and UNITA in Angola. Effectively, this made the U.S. involved in the civil wars of other countries, either supplying money, materiel, or training to whichever side was more right wing.