It’s not every case in which the Bush administration is aligned with the World Court, human-rights groups, the American Bar Association, the European Union, other death-penalty opponents, and a vicious murderer now sitting on death row against Bush’s home state of Texas and tough-on-crime groups.

Nor is it every case in which Bush-appointed Chief Justice John Roberts and his four most conservative colleagues (plus liberal John Paul Stevens) rebuff the president for making an excessive claim of presidential power.

That’s what happened on March 25, in Medellin v. Texas, a complex but fascinating case that has become a global cause celebre. The Supreme Court ruled 6-3 that neither the World Court nor the president could require Texas courts to give the murderer, a Mexican national named Jose Ernesto Medellin, a new hearing on whether he was denied a fair trial by the state’s violation of his right under a multilateral consular treaty to seek help from the Mexican consulate after his arrest. Texas courts have refused because Medellin’s attorney did not raise the state’s violation of the Vienna Convention on Consular Relations in his trial or initial appeal.

The Court issued two important rulings: 1) U.S. courts may not use vague treaty provisions to override state or federal laws without explicit authorization from Congress, a point on which Bush agreed; and 2) The president lacks the sweeping and unprecedented power that he claimed unilaterally to require judicial enforcement of such vague treaty provisions.

The Court was right on both points, in my view. But the decision, widely denounced by internationalists, does carry heavy costs. As Roberts acknowledged, it may hurt relations with foreign governments, may be seen as undermining the nation’s "commitment to international law," and may make it harder for Americans arrested abroad to get access to U.S. consular officials.

The case began in 1993 when Medellin, a Mexican national who had lived in the U.S. since preschool, was arrested in Texas in the vicious gang rape and murder of two teenage girls. He quickly confessed and was clearly guilty. But local officials violated the Vienna Convention, of which they were apparently unaware, by failing to notify Medellin of his right to seek help from the Mexican consulate.

After Medellin was convicted, sentenced to death, and lost his initial appeal, his attorney sought to overturn the verdicts based on the state’s violation of the Vienna Convention. The Texas courts refused on procedural grounds, ruling that Medellin had waited too long to raise the treaty violation and had not been prejudiced by it in any event.

The government of Mexico, heeding anti-death-penalty sentiment, later sued the United States in the World Court, formally known as the International Court of Justice. Mexico sought reversal of the convictions and death sentences of Medellin and 50 other Mexican nationals whose consular rights had also been violated. The Bush administration sided with Texas at that stage, arguing that the state’s procedural rules were consistent with international law.

The World Court split the difference in a March 2004 decision, ordering the U.S. only to give the 51 Mexicans new hearings to determine whether the consular violations had denied any of them fair trials.

At this point, the reasonable course would have been for the affected states, including Texas, to comply with the World Court’s decision. Indeed, the Bush administration, in an uncharacteristic bow to world opinion, issued a declaration in February 2005 seeking to require the states to do just that. All or most of these new hearings would have undoubtedly ended in rulings that the trials were fair and the convictions valid. Although a few defendants might have won a chance to be resentenced, none would have been released.

But the Texas courts, Legislature, and governor refused to go along. So Medellin (among others) appealed to the Supreme Court, supported by Mexico and many internationalist and liberal groups.

This presented the justices with a sticky set of problems. All nine agreed that the World Court decision "constitutes an international obligation on the part of the United States," in the words of the chief justice. The source of this obligation is the combined effect of the Vienna Convention; its "Optional Protocol," whose signatories agreed to submit disputes over consular matters to the World Court; and Article 94(1) of the United Nations Charter. The latter provides that each U.N. member "undertakes to comply with the decision of the [World Court] in any case to which it is a party."

The disagreement among the justices was over whether — as Justice Stephen Breyer and the two other liberal dissenters argued — these treaty provisions were binding in U.S. courts by virtue of the U.S. Constitution’s so-called supremacy clause. It states that "all treaties" signed by the president and ratified by two-thirds of the Senate, as well as other laws, "shall be the supreme law of the land, and the judges in every state shall be bound thereby."

This broad constitutional language notwithstanding, Supreme Court decisions dating to 1829 have held that treaties require (and allow) domestic courts to override contrary state and federal laws only if the treaties were specifically intended to be "self-executing" — that is, automatically enforceable in U.S. courts even in the absence of separate implementing legislation.

Roberts read the U.N. Charter provision that each nation "undertakes to comply" with World Court decisions as showing an intent only to require the signatories’ "political branches" to make their best efforts to comply, and not to bind their domestic courts without separate implementing legislation.

Read in isolation, this seems a stretch. But it is supported by some (not all) Supreme Court precedents. It is also consistent both with U.S. interpretations when the U.N. Charter was ratified and with the reluctance of almost all other nations to construe treaties (or World Court decisions) as automatically overriding their domestic laws.

Indeed, as Roberts stressed, the gaggle of international-law experts who took Medellin’s side in the case were unable to identify "a single nation that treats [World Court] judgments as binding in domestic courts."

Even Breyer had to concede that some treaty obligations and World Court judgments — such as "those touching upon military hostilities, naval activity, handling of nuclear material," and other "politically sensitive judgments" — are not enforceable in U.S. domestic courts. He nonetheless objected to the majority’s broad holding that World Court judgments can override state procedural rules only when "the ratifying parties to the relevant treaties … have clearly stated their intent to give [World Court] judgments domestic effect."

How, then, would Breyer choose which World Court judgments and treaties the courts should enforce? By a convoluted, multipart "context-specific" inquiry, aptly characterized by Roberts as "a grab bag" of "ad hoc judicial assessments" that would unwisely "assign to the courts — not the political branches — the primary role in deciding when and how international agreements will be enforced."

The Bush administration agreed with the majority on these points. But it also argued that the president’s February 2005 memorandum declaring that state courts should give effect to the World Court’s decision was a valid exercise of presidential power and as such binding on Texas.

Nice try. But Roberts properly held that it would usurp the legislative power of Congress to determine whether to give domestic legal force to treaties that are not self-executing.

This was a significant setback for Bush’s push to expand presidential powers. It protected the power of Congress — and diminished the powers of the courts as well as the president — to determine which treaties and World Court judgments will or will not be enforced in U.S. courts. As such, it is good for democratic governance.

"Most multilateral treaties and other international-law materials are produced by highly undemocratic processes in which authoritarian states and unaccountable political elites from democratic states play a dominant role," and the president and Senate that approved them "expected them to be unenforceable exercises in public relations," Ilya Somin, an assistant professor at George Mason Law School, wrote in the Volokh Conspiracy blog.

None of this is to understate the danger of provoking retaliation against Americans arrested overseas or the cost of reinforcing the ever-more-pervasive — if simplistic and often hypocritical — view abroad of the U.S. as an international scofflaw. The best way to limit this damage would be for Congress to step in and require state courts to enforce the World Court decision. Alas, that’s most unlikely. The second-best way would be for Texas to comply voluntarily.

In the words of Justice Stevens, who voted with the majority but did not join the Roberts opinion, "One consequence of our form of government is that sometimes states must shoulder the primary responsibility for protecting the honor and integrity of the nation."