Final answer:
President Carter responded to the Soviet troops in Cuba with public criticism and the enhancement of U.S. missile defense systems, although the troops had been there for a while due to an earlier agreement.
Step-by-step explanation:
President Jimmy Carter's response to the discovery of Soviet troops in Cuba included a two-pronged approach of domestic defense measures and public condemnation of Soviet actions. Carter criticized the Soviet Union for their military presence in Cuba, even though it was later revealed that the Soviet troops had been there for some time as part of a previously established agreement.
To bolster the United States' defensive capabilities and to counter the criticism of being strategically naive, Carter supported the development of an underground missile defense system designed to protect intercontinental missiles from potential Soviet attacks. The discovery initially led to high tensions, similar to past confrontations over Soviet actions in Cuba, most notably the Cuban Missile Crisis during President Kennedy's administration, where the presence of Soviet missiles led to a major Cold War standoff.