Luis Malpica de Lamadrid, Mexico’s consul general in Houston, talks to reporters awaiting the execution of Humberto Leal Garcia Jr. (RICHARD CARSON — REUTERS)

For the state of Texas, the execution was a matter of justice. For a majority of the Supreme Court, the decision not to stay the execution was a matter of legal obligation.

But for diplomats, the execution was at least in part a matter of international credibility.

Leal was not informed after his arrest that, under international law -- specifically under the United Nations’ Vienna Convention -- he was permitted access to Mexican consular officials. That, experts said, put the United States in breach of an international treaty, and undermined the strength of the American commitment to law.

The Obama administration filed an amicus brief last week, suggesting that Leal’s execution could make other nations less likely to respect the terms of the Vienna Convention should American citizens abroad be in need of consular assistance. It sought a stay to allow Congress to take up pending legislation to bring the United States into compliance with the treaty.

The Supreme Court ruled 5-4 to deny the stay, with the majority arguing that the court was tasked with ruling on current law, “not what it might eventually be.”

A coalition of former diplomats who had pressed for the execution to be delayed reacted with dismay, describing themselves as “disappointed” that Texas “has chosen not to comply with the clear international legal obligations of the United States.”

“The ability of the United States to secure future international agreements vital to the protection of our citizens, our national security, and our commercial interests,” they added, “depends largely on whether this nation is perceived as honoring its international commitments.”

The group included James R. Jones, a former ambassador to Mexico, and Thomas R. Pickering, a former ambassador to the United Nations and undersecretary of state for political affairs.

The immediate impact of the execution is unclear, and some experts have argued there is unlikely to be any at all.

They note that three years ago, Texas executed another Mexican national, despite the fact that he had similarly not been informed of his rights under the Vienna Convention. Then, too, there were warnings about the impact on U.S. foreign policy interests.

“Congress,” the high court’s majority wrote Thursday, “evidently did not find these consequences sufficiently grave to prompt its enactment of implementing legislation, and we will follow the law as written by Congress.”