a: communist countries
During Cold War, it started right after the WW2 ended and finished in 1989 with the fall of the Berlin Wall, the United States and the Sovietic Union were fighting over political ideology dividing the world in two: capitalist and communist blocs.
Both great powers did not get to have direct armed conflicts, instead there were indirect hostilities in third countries territories. These conflicts were sponsored by the US or the Sovietic Union: for example in the Missile Cuban Crisis, 1962, the American air force base were in the south of the island and the Sovietic missile bases were place in La Habana, the north of the island.
During the Vietnam war, the American bases were in south Vietnam and the Sovietic bases were in north Vietnam.