The crumbling of alliances between Europe and the United States could have devastating consequences. (Photo by Matt Cardy/Getty Images)
Jeffrey A. Engel is director of Southern Methodist University’s Center for Presidential History and co-author of "Impeachment: An American History." The views here are his own.

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.”

Even European unification could not replace American oversight. Without American stabilization, one ranking State Department official lamented in 1990, “European squabbling would undermine political and economic structures like the E.C. [European Community],” leading to resumption of “historic conflicts.”

That observation rings prescient a quarter-century later. Buffeted by economic head winds and humanitarian crises, and in some circles a sense that traditional European culture is under assault, nationalism is once more on the rise — as the new American administration disengages.

Nationalism pulsates from the White House as well, unlike anything seen since Americans assumed their postwar role. If Trump continues dismissing European climate concerns, abusing the notion of collective security and complaining that European allies have not done their fair share for their common defense, there may not be an American presence to keep the peace.  Allies derided, insulted and even physically shoved aside will not tolerate America’s presence for long, especially if they come to doubt Washington’s commitment to their protection.

Which he has given them every reason to doubt.  After failing to offer even a hint of reassurance during his initial European trip in May, Trump subsequently offered forced and halfhearted pledges of fidelity to NATO’s critical mutual defense pact. Few have profited from relying on this president’s fidelity, and his continued fascination with Russia leaves no reasonable European policymaker fully confident of American support during any future crisis with Moscow.

Trump is thus fueling the very forces that previous generations of American policymakers feared as much — if not more — than the Soviets, and the real causes of Europe’s historic inability to keep its own peace. Nationalism is on the rise. Britain voted to leave the European Union. France, Austria and the Netherlands nearly elected proud xenophobes. Poland and Hungary have. More ominously, Europeans are re-arming.  Sweden has recently reintroduced military conscription. Norway expanded its own. The European Union announced plans for a new headquarters for military planning and coordination, outside of NATO control or integration.

The president gleefully takes credit for this enhanced military spending, incorrectly thinking that his badgering pried open European purse strings. His erosion of European trust played a far greater role in what is actually an ominous development. “We Europeans must really take our destiny into our own hands,” Germany’s Angela Merkel declared after contentious meetings with Trump. “The times in which we could rely fully on others — they are somewhat over.” Similar statements by former European leaders, German especially, did not end well. Cries of destiny led inexorably to cries for war.

Rows of stark-white American tombstones in manicured European fields attest to the hard-won cost of the wisdom Trump rejects. Yes, it is conventional wisdom of the type his administration and supporters abhor.  But not all conventional wisdom is wrong. Renewed American nationalism, economic and otherwise, may in the short term produce domestic prosperity. Improved Russian-American relations may prove similarly beneficial, although history offers few examples of a nation made great by isolation and appeasement. Far more troubling is the president’s rejection of what every one of his predecessors preached since 1945 and the lesson they drew from the past. Absent American leadership, the ancient hatreds and contemporary desires of the old world may once more consume the new.