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How did the dual-key system control the use of U.S. nuclear weapons in Europe?

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Flag of NATO featuring a white compass rose emblem with four lines radiating from the center on a blue background. — North Atlantic Treaty Organization Flag of the North Atlantic Treaty Organization (NATO).

The dual-key system was a mechanism implemented by NATO to manage the deployment and potential use of American nuclear weapons in Western Europe during the Cold War.1

Under this arrangement, battlefield and theatre nuclear weapons were provided by the United States to allied forces, but their activation required the authorization of both the host nation and the United States. This dual-control structure effectively granted each party a veto over the use of the weapons, ensuring that neither the host country nor the U.S. could unilaterally initiate a nuclear strike. This system was a key component of NATO’s "flexible response" strategy, which sought to provide a measured military response to threats without necessarily escalating immediately to an all-out nuclear exchange.1