The shift comes after the high court ruled on Friday that the president exceeded his authority by using the International Emergency Economic Powers Act (IEEPA) to implement tariffs without congressional approval.

Trump criticized the justices as a "disgrace to the nation" and immediately invoked a separate legal mechanism to apply a 10% levy, which he then raised to 15% via a Truth Social post on Saturday.

While the president claimed the new 15% rate takes effect "immediately," official White House fact sheets had previously scheduled the initial 10% levies to begin at 12:01 a.m. ET on Tuesday, February 24.

The aggressive trade policy, Trump said, aims to revitalize U.S. manufacturing, with the government already having collected at least $130 billion in tariffs through previous use of the IEEPA.

An economist at Capital Economics suggested the sudden increase to 15% likely stems from a need to generate more revenue than the 10% rate would have provided.

The decision creates a uniform trade barrier that affects all U.S. trading partners. Economists noted that while the European Union and Japan find themselves back in a familiar position of high-tension trade relations, the United Kingdom faces a significant setback.

The UK had previously anticipated a more advantageous 10% rate, but the move to 15% appears unavoidable under Section 122, which requires tariffs to be applied in a non-discriminatory manner.

Wall Street remains cautious regarding the long-term implications of the administration's "fraught" path forward. Analysts at JPMorgan stated in a note to clients that trade uncertainty will remain elevated, though they maintain a base case that average tariff rates will eventually settle between 9% and 10%.