Long-threatened tariffs from U.S. President Donald Trump have plunged the country into a trade war abroad — all while on-again, off-again new levies continue to escalate uncertainty.

U.S. tariffs on Canada have put experts at odds over how appropriate the action was, particularly given the unaddressed demand side of the fentanyl epidemic.

Trump, who imposed a wave of tariffs on China in his first term, has slapped another 20% duty on a wide swath of Chinese goods this year. Beijing, the top buyer of American agricultural products, recently imposed retaliatory tariffs of 10% to 15% on a range of U.S. exports like chicken, corn, soybeans and beef.