The U.S. Supreme Court has begun considering the legality of tariffs President Donald Trump has imposed on nations around the world, including South Korea.

Oral argument began on Wednesday at the Supreme Court in Washington. 

The key question in the nearly three-hour hearing was whether Trump’s invocation of the 1977 International Emergency Economic Powers Act provided a legal basis for the tariffs.

Solicitor General John Sauer, representing the U.S. government, told the justices that Trump had invoked emergency powers because he believed the United States was on the brink of an economic and national security crisis.

Neal Katyal, representing small businesses challenging the tariffs, countered that “a tariff is a tax” and that the Constitution grants the power of taxation only to Congress.

Lower courts, including a specialized trade court, a district court in Washington and a federal appeals court, have ruled in favor of the challengers, finding that Trump lacked the legal authority to impose tariffs under the emergency powers law.