9:1 a.m. Leave it to China to crash Jerome Powell’s party—and send Dow Jones Industrial Average futures into negative territory.

Dow futures had been trading higher ahead of Federal Reserve Chair Jerome Powell’s speech at Jackson Hole later this morning. Then China announced that it will impose new tariffs on $75 billion worth of U.S. products, and start levying penalties on cars again.

And with that, futures turned lower. Dow futures have fallen 121 points, or 0.5%, while S&P 500 futures have declined 0.4%, and Nasdaq Composite futures have dropped 0.7%.

Of course, investors will still be waiting to see what Powell has to say. Some Fed presidents had been making fairly hawkish remarks in the run-up to Powell’s speech—and whether the new announcement forces him to change his tone is anyone’s guess, “We don’t think so,” writes NatAlliance Securities’ Andrew Brenner. “Powell is already concerned about a trade war that he can’t control.”

Powell was already in a tough enough sport before today, given the mixed signals being sent about the U.S. economy. The yield curve has inverted—this time, the two-year and 10-year portion of the curve—which would usually get everyone worrying about a looming recession but might not be as reliable this time around. U.S. manufacturing is slowing—witness the manufacturing purchasing managers index, which slipped below 50 in August, according to IHS Markit—but the U.S. consumer appears to spending.

Against that backdrop, you have the trade war, which may or may not be responsible for those mixed signals. It also makes you wonder just how much of a cushion monetary policy can provide. The uncertainty U.S. companies are dealing with is not due to trouble accessing money—there seems to be plenty available—but decision making around the supply chain and other impacts from the trade war.

No wonder the Jackson Hole meeting is focused on the challenges facing monetary policy.

Write to Ben Levisohn at Ben.Levisohn@barrons.com