Castro took every opportunity he could to influence American opinions. From Guerrilla Prince by Georgie Anne Geyer:
It was with these radical young Americans that Castro was most successful in helping the North Vietnamese. Castro never had more than some hundreds of troops in Vietnam, although in the 1970s he had up to a thousand workers and technicians there and he himself visited Vietnam in 1973; his aid to North Vietnam was other than combat fighting. He made Havana a meeting place (the only meeting place) where groups like the American radical Weathermen could meet directly with the North Vietnamese. As early as 1963, he had established the Cuban Committee of Solidarity with South Vietnam, the first of its kind in the world. Throughout the 1960s, he organized constant meetings between the young Americans and Huynh Van Ba, the Vietcong's chief representative in Havana, who instructed the Americans how to organize more antiwar demonstrations at home, to emphasize the number of American casualties and the number of planes being shot down, and to encourage draft resistance. Van Ba was very insistent on one point that Castro understood better than anyone in the world: the Americans should be careful not to use the word "Communism," just as the revolutionary movements in Cuba and Vietnam had avoided it during the first stages of their revolutions.
