President Trump said Thursday he is slapping a 10 percent tariff on $300 billion in Chinese imports as of Sept. 1, saying President Xi Jinping failed to fulfill key promises. Mr. Trump said the levy is on top of the heftier tariffs affecting more than $250 billion in imports. In a series of tweets, the president said Mr. Jinping has failed to block shipments of fentanyl to the U.S, as he’s promised, and hasn’t purchased U.S. farm products. Mr. Trump had touted both pledges as signs of incremental progress when they occurred, though now he says he’s disappointed with Mr. Xi. The China tariff announcement immediately reversed a positive day on Wall Street, turning a 250 point-plus gain in the Dow Jones index Thursday into a loss of more than 100 points within 15 minutes of the White House announcement. Both the broader S&P 500 and the tech-heavy Nasdaq indexes also fell back sharply. Even as he slammed China, Mr. Trump held out hope for a trade deal after “constructive” talks between both sides in Shanghai this week. Mr. Trump said his new levies will take effect right as Chinese officials travel to Washington in September to resume talks. “We look forward to continuing our positive dialogue with China on a comprehensive Trade Deal, and feel that the future between our two countries will be a very bright one!” Mr. Trump wrote.