Final answer:
The nation that stepped in to attempt to stop communist aggression in Vietnam after the French defeat at Dien Bien Phu was the United States. This marked a shift from supporting the Viet Minh against Japan to preventing the spread of communism, leading to direct U.S. military involvement in Vietnam.
Step-by-step explanation:
After the French were overrun at Dien Bien Phu, it was the United States that stepped in and attempted to stop communist aggression in Vietnam. The Dien Bien Phu defeat in 1954 marked the end of French colonial efforts in Indochina and opened the door for increased American involvement. Initially, the U.S. had supported the Viet Minh in their struggle against the Japanese during World War II. However, post-war, the U.S. shifted its support to France, fearing the spread of communism. After France's withdrawal, America's commitment to containing communism prompted it to take a more direct role, and they provided significant military and financial support to the non-communist South Vietnamese government.
Despite the hope of a united Vietnam, the reality of the Cold War dynamics led the United States to bolster South Vietnam, directly leading to the escalation that turned into the Vietnam War. This involvement was largely fueled by the U.S. policy of containment, which aimed to prevent the further spread of communism following the Korean War.