NATO was established in 1949 with the purpose of deterring the Soviet Union and other communist states allied with Moscow.
The aggressive and expansionist nature of the military bloc was evident despite hypocritical defensive rhetoric – soon after the creation of NATO, US strategists came up with Operation Dropshot that implied full-scale war in Europe and nuking Soviet territories with 300 bombs. This plan never came to life thanks to advances in Soviet technology, yet the Washington-backed NATO remained Moscow's archenemy until the end of Cold War, and beyond.
At the end of the 1980s, the White House offered the Kremlin a promise to stop NATO expansion. Yet, after the dissolution of Soviet Union, the West pressed on and soon new states joined the anti-Russian bloc. When asked about the promise given to Kremlin, US officials claim that these words were never codified and that this promise was given to the head of state that no longer exists – and this makes them idle.
See Sputnik's interactive map to learn how the North Atlantic Treaty Organization (NATO) has expanded.