The term Cold War defines the political, ideological, strategic, and military tensions between the United States and its North Atlantic Treaty Organization allies (the Western bloc) and the Union of Soviet Socialist Republics (USSR) and its Warsaw Pact allies (the Eastern bloc) from 1945 to 1991. Occasionally, the tensions between the East and the West were acute, but there was never a “hot,” armed large-scale conflict. Considering its “cold” nature, the Cold War was based on espionage and surveillance rather than on overt military action, with both sides using different surveillance systems—spy networks, radar stations, reconnaissance crafts, and satellites.