“International linkages have risen dramatically over recent decades,” Mark Carney, the head of the Bank of England, said at the Jackson Hole gathering, an annual meeting hosted by the Federal Reserve Bank of Kansas City. And because America’s currency and debt markets are so central to the global financial system, the nation’s political and economic dramas guide the world’s.

“The global financial cycle is a dollar cycle,” Mr. Carney said. Bank of England research indicates that increases in America’s policy interest rate have twice the effect on foreign growth that they did back in the 1990s, even though America now makes up a smaller share of the global economy, Mr. Carney said.

When the Fed moves interest rates or Mr. Trump ramps up trade tensions, it echoes across the world through currency repricing and slower growth.

There are hazards to such an integrated global monetary and financial system. It leaves central banks with low interest rates to begin with, and then little ability to diverge from their trading partners’ monetary policy settings. At the first sign of trouble, many nations may lower rates in tandem.

That seems to be happening now. As the United States, the eurozone and Japan reorient toward rate cuts and other forms of monetary economic help this year, central bankers across emerging markets have slashed their own borrowing costs. Monetary authorities could enter the next recession with relatively little ammunition, heightening the risk that a garden-variety economic slowdown could turn into a drawn-out, painful global slump with widespread costs to jobs and prosperity.

And while the global economy’s brittleness is rooted in slow-moving economic fundamentals, Mr. Trump’s trade war could be the spark that sets off the time bomb.

Mr. Trump looks at a tightly intertwined global economy and sees a winner-take-all game in which the United States can and should prosper at the expense of other countries. He has criticized the Federal Reserve for not cutting rates more quickly, saying the central bank is putting the United States at a disadvantage to other nations that are ushering in low rates.