Donald Trump’s election was a victory for an enemy the United States defeated almost three decades ago — the Soviet Union. Mere months into office, he has accomplished what every Cold War leader in the Kremlin desired: a weakening of America’s transatlantic military, political and economic ties that has left Europe ripe for Moscow’s dominance.

But looking at the wilting of America’s influence and alliances, especially the weakening of NATO, solely through the lens of long-term Russian aspirations misses the bigger picture. Trump’s fulfillment of Kremlin aspirations also violates the central tenet of America’s foreign policy since 1945: The United States must stay actively involved to keep Europe stable. Trump instead intends to leave Europe to battle its history alone — a move that threatens the security of the continent, and our own.

American leaders weaned in the shadow of two world wars thought their presence across the ocean was less a matter of altruism than common sense. History told a straightforward tale: The United States won the first world war, went home and a generation later had to do it all over again. Peace prevailed when they stayed. The lesson? The continent’s inhabitants could not take care of themselves. The new world must babysit the old.

Generations of postwar American strategists imbibed this dogma, regardless of party or politics. “The history of the past two hundred years in Europe showed that Western Europe would tear itself to pieces” without outside supervision, Republican Secretary of State John Foster Dulles explained in the early 1950s. His Democratic successor, Dean Rusk, toed the same line. “Without the visible assurance of a sizeable American contingent,” he explained, “old frictions may revive, and Europe could become unstable once more.” Advisers to Nixon, Ford, Carter and Reagan all said the same.

Their immediate successors, trained to see America’s presence as vital to Europe’s stability, and thus to peace, consequently approached Cold War victory cautiously, worried that it might produce calls for NATO’s disbandment as Congress salivated over a “peace dividend” of reduced military spending. Never, President George H.W. Bush’s inner circle thought, had it been more important to recognize that NATO, and the American presence it entailed, was as much about Europe as the Soviet Union. “The basic lesson of two world wars was that American power is essential to any stable equilibrium on the continent,” national security adviser Brent Scowcroft warned. “The postwar era’s success is founded on recognition of this fact.” ...