American financial markets plunged Friday after President Trump opened a new front in his already-broad trade war, threatening fresh tariffs on Mexico unless its government helps keep illegal immigrants from crossing the southern U.S. border.

The blue-chip Dow Jones Industrial Average tumbled 354 points, or 1.4%, to 24,815, the lowest since late January. The broader S&P 500 fell 1.3% and the tech-heavy Nasdaq composite dropped 1.5% as the moves fueled the risk of an economic downturn. Analysts warned the duties would undercut automakers who rely on Mexican production, and manufacturers cautioned that linking immigration challenges to trade will create “a Molotov cocktail of policy.”