In the year since Donald Trump was elected president, the economy and markets have been on a roll. Stocks have set one record after another, unemployment has dropped sharply and the U.S. enjoyed its strongest six months of economic growth since 2014.

Mr. Trump thinks he knows why: “The reason our stock market is so successful is because of me,” he declared earlier this week.

But Mr. Trump should be giving thanks, not taking credit. The entire global economy is picking up steam, and foreign stocks are outperforming American markets. This suggests the U.S.’s good fortune is due less to Mr. Trump’s presence than to a broader, global trend. Years of highly stimulative monetary policies by central banks have finally overcome various postcrisis headwinds holding back growth.

So Mr. Trump is a lucky man. The question is, can he make his luck last? That will require translating what may be a short-lived upswing into permanently faster growth, and as the fight over U.S. tax cuts demonstrates, that is no easy task.

The disappointing pace of global growth over the past decade reflects both structural drags such as aging populations and the headwinds that followed the U.S. financial crisis in 2008 and Europe’s sovereign debt crisis in 2012. Central bankers responded to both with bond buying and zero interest rates.