The continent became a battlefield of the Cold War in the late 20th century. Some democratically elected governments of Argentina, Brazil, Chile, Uruguay, and Paraguay were overthrown or displaced by military dictatorships in the 1960s and 1970s. To curtail opposition, their governments detained tens of thousands of political prisoners, many of whom were tortured and/or killed on inter-state collaboration. Economically, they began a transition to neoliberal economic policies. They placed their own actions within the US Cold War doctrine of "National Security" against internal subversion. Throughout the 1980s and 1990s, Peru suffered from an internal conflict. South America, like many other continents, became a battlefield for the superpowers during the Cold War in the late 20th century. In the postwar period, the expansion of communism became the greatest political issue for both the United States and governments in the region. The start of the Cold War forced governments to choose between the United States and the Soviet Union.