President Trump has many bad ideas. Reconsidering America’s role in NATO isn’t one of them.

NATO, a military alliance, was formed specifically to prevent the Soviet Union from dominating Europe, whose principal powers — Germany, France, Italy and Britain — had been so devastated by World War II that they were vulnerable to Soviet coercion, subversion or conquest. NATO also became a vehicle for rehabilitating the Axis powers — Germany and Italy — under the victors’ tutelage.

America had an enduring interest in ensuring that the Continent not fall under the domination of a single, capable, hostile power: That could pose a serious threat to America. The Truman administration was clear on this point: The main purpose of stationing American military forces in Europe in the early 1950s was to stay long enough to right the balance of power, not to stay forever.

By the 1960s, the balance was restored. Western Europe’s economies were booming; Britain and France had become nuclear powers; German militarism had been tamed, even as a new, large modern army emerged in West Germany. During the Vietnam War, America was so free of worry about the Soviet threat that it essentially milked its conventional forces in Europe to support its war in Indochina. Its European allies contributed nothing to America’s effort in Vietnam. By 1968, with the collapse of Czechoslovakia’s government and army, the Soviet-controlled Warsaw Pact alliance also appeared less capable.

In the early 1970s, Senator Mike Mansfield, a Montana Democrat, led an effort to cut the American troop presence in Europe. It lost momentum in part because the Soviet Union conducted an ill-fated military buildup, which contributed to its economic failures in the 1980s. Senator Mansfield’s campaign was also opposed by the Europeans, who preferred to keep their American security blanket, and by President Richard Nixon and his chief foreign affairs adviser, Henry Kissinger, who rejected what he considered congressional meddling in foreign policy. Oddly, troop reductions in Europe would have been consistent with the “Nixon Doctrine,” which called explicitly for allies to do much more in their own defense.