The S&P 500 and the tech-heavy Nasdaq turned in their worst performances this year on Monday after China retaliated in kind for massive U.S. tariff increases, ignoring threats from President Trump.

One of the most widely followed gauges of U.S. stock market performance, the S&P tumbled 2.4%, the biggest decline since early December, as optimism about a trade agreement between the world’s two largest economies gave way to fears of an escalating trade war. The Nasdaq sank 3.4%, and the blue-chip Dow Jones industrial average fell 617 points, or 2.4%, its worst day since early January.