The U.S. Supreme Court struck down President Donald Trump's sweeping global tariffs, undercutting his signature economic policy and delivering his biggest legal defeat since he returned to the White House.

Voting 6-3, the court said Trump exceeded his authority by invoking a federal emergency-powers law to impose his "reciprocal" tariffs across the globe as well as targeted import taxes the administration says address fentanyl trafficking.

The justices didn't address the extent to which importers are entitled to refunds, leaving it to a lower court to sort out those issues. If fully allowed, refunds could total as much as $170 billion — more than half the revenue Trump's tariffs have brought in. 

Dissenting Justice Brett Kavanaugh said the refund process was "likely to be a 'mess,'" as was acknowledged at oral argument. Justices Clarence Thomas and Samuel Alito also dissented.

The White House has said it will quickly replace the levies using other legal tools, though the fall-back options tend to be either more cumbersome or more limited than the wide-ranging powers Trump asserted under the International Emergency Economic Powers Act.