WASHINGTON — The Supreme Court dealt a defeat to President Bush yesterday and ruled he does not have the “unilateral authority” to force state officials to comply with international treaties. Chief Justice John Roberts said the Constitution gives the president the power “to execute the laws, not make them.” Unless Congress passes a law to enforce a treaty, the president usually cannot do it on his own, Justice Roberts said.

The case arose from an unusual dispute and an unexpected intervention by Mr. Bush. But the justices used it to make a strong statement about the limits of presidential power.

The International Court of Justice in The Hague, acting on a suit by Mexico, ruled America had failed to carry out its treaty duty to inform a native country when one of its citizens was arrested and charged with a serious crime. The decision pointed to 51 Mexican nationals who were under death sentences in Texas, California, and several other states.

It was unclear how this ruling could be enforced. But in a surprise move, Mr. Bush in 2005 told Texas officials they must reopen and reconsider the cases of the Mexican-born murderers on Death Row. Mr. Bush, a former Texas governor, said he was acting “pursuant to the authority vested in me as president by the Constitution and laws of the United States.”

Texas officials refused to go along with Mr. Bush’s order, and they fought him in court.

In yesterday’s 6-3 decision, the court said the Texas prosecutors were right, and Mr. Bush was wrong.

Justice Roberts said the treaty known as the Vienna Convention was not “binding federal law” because Congress had not passed a law to enforce it. And the president cannot do this on his own, the chief justice said.

“The president’s authority to act, as with the exercise of any governmental power, must stem either from an act of Congress or from the Constitution itself,” Justice Roberts said.

“We therefore conclude, given the absence of congressional legislation, that the non-self-executing treaties at issue here did not expressly or impliedly vest the president with the unilateral authority to make them self-executing,” he said.

Justices Antonin Scalia, Anthony Kennedy, Clarence Thomas, and Samuel Alito Jr. joined Justice Roberts’ opinion. And Justice John Paul Stevens concurred in the result.

The dissenters, led by Justice Stephen G. Breyer, took the view that treaties become part of American law and therefore, must be followed by state and federal officials.