White House officials on Friday said the United States would begin withdrawal from a landmark 1987 nuclear arms-control pact with Russia, asserting the Russians have violated it for years.

The decision suspends American obligations under the treaty, and terminates the accord unless Russia comes into compliance within six months. But with Russia asserting it is complying, the outlook for preserving the treaty, regarded as one of the most important in the history of preventing nuclear war, is bleak.

Here is a look at the origins of the accord, known as the Intermediate-Range Nuclear Forces Treaty, and the possible impact of its demise.

What is the I.N.F. Treaty and how did it come about?

The treaty resolved a crisis of the 1980s when the Soviet Union deployed a missile in Europe called the SS-20, capable of carrying three nuclear warheads. The United States responded with cruise and Pershing II missiles based in Europe.