From Wikipedia

Cuban Missile Crisis

Part Two/Two

The Americans feared the Soviet expansion of Stalinism, but for a Latin American country to ally openly with the USSR was regarded as unacceptable, given the Soviet-American enmity since the end of World War II in 1945. Such an involvement would also directly defy the Monroe Doctrine, a United States policy which held that European powers should not interfere with states in the Western Hemisphere.