WASHINGTON (AP) -- China retaliated against President Donald Trump's tariffs with an additional 15% tax on key American farm products, including chicken, pork, soybeans and beef.

The escalating trade tensions punished U.S. markets Monday as investors fearful of the damage from from Trump's trade wars put their money elsewhere.

The Chinese tariffs, announced last week, were a response to Trump's decision to double the levy on Chinese imports to 20% on March 4. China's Commerce Ministry had earlier said that goods already in transit would be exempt from the retaliatory tariffs until April 12.

Imposing tariffs on imports is a key part of Trump's agenda. He believes the import taxes can raise money for the Treasury, protect American industries and pressure foreign countries to do what he wants in a range of issues, including immigration and drug trafficking.

On Wednesday, Trump is set to remove exceptions on 25% steel tariffs he imposed in 2018 -- effectively raising the taxes -- and raise his levy on aluminum from 10% to 25%.

In a bewildering series of announcements last week, Trump slapped tariffs on Canadian and Mexican imports, then delayed many of them for 30 days. Next month, he could plaster "reciprocal tariffs'' -- meant to raise U.S. tariffs to match higher tariffs imposed by foreign countries -- on a wide range of imports from around the world.

Economists warn that tariffs raise prices for consumers and make the U.S. economy less efficient as protected American companies have less incentive to innovate.

There's also the threat of retaliation, and farmers, who are among Trump's most loyal supporters and also have vigilant defenders in Congress, make a tempting target.

China also hit American farm products during the president's first-term trade wars. U.S. farm sales to China plummeted, then recovered after the two countries reached a truce in January 2020 and Beijing promised to buy more from U.S. farmers. American farm exports to China peaked at $38 billion in 2022, then fell to $29 billion in 2023 and $25 billion last year. In January, they were down 56% from a year earlier, according to the U.S. Department of Agriculture.

During his first term, Trump spent tens of billions of dollars in taxpayer money to compensate farmers for lost exports.