For decades, global commerce abided by tariff rates agreed to by the U.S. and 122 other countries during the 1980s and 1990s. On Wednesday, Trump detonated that arrangement, saying that other countries had exploited the system and “ripped off’’ the United States for years, causing its once-mighty manufacturing base to shrink.

U.S. financial markets responded with significant losses after President Donald Trump unveiled his tariff plans.