The Supreme Court’s decision invalidating President Donald Trump’s tariffs sends a clear and crucial message: The justices will not be a simple rubber stamp approving presidential actions. In the first year of Trump’s new term, 24 challenges to presidential actions came to the court, almost all on its emergency docket. In 22, the justices ruled in favor of the president. But Friday’s 6-3 decision striking down his tariffs is a huge victory for separation of powers and the rule of law.

The importance of tariffs to Trump, and their consequences for the world, cannot be overstated. The president said that their invalidation “would be a total disaster for the country” and “would literally destroy the United States of America.” In its petition to the Supreme Court, Solicitor Gen. D. John Sauer said “the tariffs are promoting peace and unprecedented economic prosperity” and “pulling America back from the precipice of disaster, restoring respect and standing in the world.”

Trump has treated tariffs as something he can impose or rescind at will. But not anymore. The court, in an opinion by Chief Justice John G. Roberts Jr., ruled that Trump lacked the power to impose tariffs, based on a basic constitutional principle: Congress, not the president, has the power to impose taxes, and tariffs are taxes. Roberts began his opinion by explaining this and quoted a decision from 1824, that the “power to impose tariffs is ‘very clear[ly] . . . a branch of the taxing power.’ As he stated, “A tariff, after all, is a tax levied on imported goods and services.”

The focus of the decision is on whether a federal statute, the International Emergency Economic Powers Act (IEEPA) authorizes the president to impose tariffs. The IEEPA, however, does not mention tariffs, but rather authorizes the president to “regulate ... importation” in order to “deal with any unusual and extraordinary threat.”

Roberts, joined by Justices Sonia Sotomayor, Elena Kagan, Neil M. Gorsuch, Amy Coney Barrett and Ketanji Brown Jackson, emphatically concluded that the law does not provide the president authorization to impose tariffs. Roberts added that the “IEEPA’s grant of authority to ‘regulate ... importation’ falls short. IEEPA contains no reference to tariffs or duties. The Government points to no statute in which Congress used the word ‘regulate’ to authorize taxation. And until now no President has read IEEPA to confer such power.”

This is clearly correct. The most basic principle of interpreting statutes is that courts must follow the plain language of the law. Nothing in the IEEPA says a word about tariffs. If Congress is to delegate its power to raise taxes, including tariffs, it must do so explicitly. Also, as Jackson argued in her concurring opinion, there is nothing in the legislative history of the IEEPA that indicates it was intended to give the president broad authority to impose tariffs.

Much of the 160 pages of opinions on this case are a fascinating debate among the justices about a principle of law created by the court just a few years ago: the major questions doctrine, which says that a federal agency cannot act on a major question of economic or political significance without clear guidance from Congress. The Supreme Court used it in 2022 to strike down the Biden administration’s requirement that those in workplaces with more than 100 employees be vaccinated against COVID or regularly tested. In 2023, the court invalidated President Biden’s student loan relief program because it involved a major question of economic and social significance without clear guidance from Congress.

Both of these cases were 6-3 decisions with the conservative justices in the majority. In the tariffs case, the justices split 3-3-3 as to whether they violated the major questions doctrine. Roberts, joined by Gorsuch and Barrett, said that tariffs are obviously a major question of economic and political significance and Congress has not given clear authority to the president. Quite significantly, they rejected Trump’s position — and that of the three dissenters — that the major questions doctrine does not apply in the area of foreign policy.

The three liberal justices — in an opinion by Kagan, joined by Sotomayor and Jackson — did not join the part of the court’s decision invalidating the tariffs based on the major questions doctrine. They dissented in every prior case about the major questions doctrine and disagree overall with its existence. Although it is understandable why they did not want to use it, and why it was unnecessary for them to strike down the tariffs, the doctrine exists even if these justices dislike it and it helps to explain why under current law the tariffs are invalid.

In the long term, these justices should be willing to use the major questions doctrine as a check on the Trump administration.

The Supreme Court’s tariffs decision certainly leaves many questions unresolved. Most important, must there now be refunds of the illegally imposed tariffs and, if so, how will this be paid for and implemented? The court did not discuss that part at all.

The greatest significance of the tariffs decision is that it shows a court willing to say no to Trump on a significant issue. If the guardrails of democracy are to hold with a president who believes, in the words of his Chief of Staff, Susie Wiles, that he can do literally anything, the courts are an essential and perhaps the only check on the president.

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Erwin Chemerinsky is the dean of the UC Berkeley Law School.

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