When Donald Trump recently declared that “Americanism, not globalism, will be our credo,” he was expressing the kind of sentiment that animates not only his new Republican coalition, but nationalists everywhere. From the leaders of Russia and China to the rising European parties hostile to an open Europe, these nationalists are linked by a belief that in all matters of policy, their nation should come first.

This world-wide turning inward, however, comes in a period when countries are more beholden than ever to one institution, the U.S. Federal Reserve. Every hint of a shift in Washington’s monetary policy is met with a sharp response by global markets, which in turn affect the U.S. economy more dramatically than ever.

The Fed has been forced to recognize that it can no longer focus on America alone. When the Federal Open Market Committee voted in January 2015 to hold interest rates steady, its official statement explicitly noted, for the first time, that it was factoring “international developments” into its decisions. Since then the Fed, including this week, has frequently cited international threats, from Brexit to China, as reason to continue with hyper-accommodative monetary policy.

Though Mr. Trump argues that America must tend to its own affairs because it is weak, the Fed’s evolving role shows the limits of this argument. The U.S. may have slipped as an economic superpower, falling to 23% of global GDP from 40% in 1960. But as a financial superpower Washington has never been more influential. Forecasts of the dollar’s downfall have completely missed the mark.

Since the 15th century the world has had six unofficial reserve currencies, starting with the Portuguese real. On average they have maintained their leading position for 94 years. The dollar succeeded the British pound 96 years ago, and it has no serious rival in sight.