Final answer:
The doctrine involving the application of counterforce by the United States against communist expansion is known as the policy of containment, exemplified by the Truman Doctrine and the Reagan Doctrine. These policies involved support for countries resisting communist influence, such as aid to Greece and Turkey, and later, support for regimes opposing communist forces worldwide.
Step-by-step explanation:
The doctrine that the United States would apply occasional counterforce to attempts at communist expansion is commonly referred to as the policy of containment. This strategy was a central tenet of U.S. foreign policy during the Cold War era. It was first articulated in the Truman Doctrine, which declared that the U.S. would support free peoples resisting subjugation by armed minorities or outside pressures. An example of this policy in action was the U.S. aid provided to Greece and Turkey to prevent the spread of communism.
Later, the Reagan Doctrine became a guiding force in President Reagan's foreign policy, suggesting that the U.S. must support the armed forces of any regime waging war against Communist forces. Despite challenges, particularly in Latin America and Southeast Asia, the U.S. persisted in employing this doctrine to contain the influence of the Soviet Union and prevent the spread of communism, as demonstrated by the escalation in Vietnam and interventions in various countries.