In issuing a deliberately narrow ruling yesterday in a controversial case involving whales and the U.S. Navy, the Supreme Court strongly indicated that it intends to defer to the military in future disputes pitting national security against environmental concerns.

The justices voted 6 to 3 to lift restrictions on the Navy's use of sonar off the Southern California coast, backing the military in a longstanding battle over whether anti-submarine training harms marine mammals. Environmentalists say the exercises disrupt habitats and leave the mammals with permanent hearing loss and decompression sickness. But the Navy argued that the training missions are essential to detecting a new generation of "quiet" submarines deployed by China, North Korea and other potential adversaries.

"We do not discount the importance of plaintiffs' ecological, scientific, and recreational interests in marine mammals," Chief Justice John G. Roberts Jr. wrote in the first decision of the court's current term. "Those interests, however, are plainly outweighed by the Navy's need to conduct realistic training exercises to ensure that it is able to neutralize the threat posed by enemy submarines."

Roberts was joined by Justices Samuel A. Alito Jr., Anthony M. Kennedy, Antonin Scalia, and Clarence Thomas. Justice John Paul Stevens agreed with the ruling, while Justice Stephen G. Breyer agreed in part but also dissented. Justice Ruth Bader Ginsburg issued a full dissent, which was seconded by Justice David H. Souter.

Although the majority tailored its decision on narrow legal grounds and indicated that future environmental disputes will be decided on a case-by-case basis, the court made sweeping statements of deference to military judgments. Roberts unquestioningly accepted the assertion of top Navy officers that the exercises "are of utmost importance to the Navy and the Nation," writing that "the proper determination of where the public interest lies does not strike us as a close question."

The Navy's argument had raised broad questions about the military's obligation to obey environmental laws as well as the constitutional separation of powers. The Bush administration, relying on positions it has advanced in national security cases since the Sept. 11, 2001, attacks, had raised fears among environmentalists by trying to exempt the military from several environmental laws and essentially override a court decision that imposed restrictions on the exercises.

Roberts deliberately skirted such broad arguments, writing that the court would not "address the underlying merits" of the Navy's assertions and that "of course, military interests do not always trump other considerations, and we have not held that they do."

Still, Navy leaders hailed the decision, saying it would enable them to continue carrying out vital training missions before units are deployed to the Middle East and other global hot spots. "This case was vital to our Navy and nation's security, and we are pleased with the Supreme Court's decision," Navy Secretary Donald C. Winter said in a statement. "We can now continue to train our sailors effectively, under realistic combat conditions, and certify our crews 'combat ready' while continuing to be good stewards of the marine environment."

Environmentalists offered a mixed appraisal, with some suggesting that the Navy could take additional precautions to protect endangered species without jeopardizing national security. The majority justices "decided to turn a deaf ear to the substantive issues and scientific evidence presented in this case," said Nathan Herschler, a legal fellow at the International Fund for Animal Welfare.

The Natural Resources Defense Council, which sued the Navy over the exercises, took comfort in the decision's narrow language and pointed out that four of the six restrictions imposed by a lower court remain. The Navy had challenged two of the restrictions. Environmental groups had said that an adverse ruling could free the government to take other potentially harmful actions without studying their effects on the environment. But Eric Glitzenstein, a lawyer who filed a brief on behalf of the groups, including the Sierra Club, said those concerns have been mollified. "I don't see this as having dire effects on environmental litigation, particularly as it pertains to wildlife," he said.

At issue in the case was the Navy's use of loud, mid-frequency sonar during 14 anti-submarine training exercises off the Southern California coast that began in February 2007 and are scheduled to end in January. The defense council sued in federal court to stop or modify the use of sonar. The Navy maintains that the coastal waters are the best place to conduct the exercises, but the habitat is shared by 37 species of marine mammals, including whales, dolphins and sea lions.

A federal court in California issued an injunction requiring the Navy, among other restrictions, to shut down the sonar when a marine mammal was spotted within 2,200 yards of a vessel and to power down the sonar when certain water conditions were detected. An appeals court upheld the injunction, and the Navy took the case to the Supreme Court.

Roberts noted in his opinion that the two sides strongly disputed the extent to which the exercises harm the marine mammals or disrupt their behavioral patterns.

The Navy contended that to obtain a preliminary injunction, the plaintiffs had to demonstrate a likelihood of irreparable injury to the mammals, not simply the possibility, and that the alleged injuries were too speculative to justify the injunction.

The majority agreed and decided the case on that narrow ground, ruling that the standard used by the U.S. Court of Appeals for the 9th Circuit in upholding the preliminary injunction was "too lenient" and said the plaintiffs needed to demonstrate that irreparable injury was likely without the restrictions.

"Even if plaintiffs have shown irreparable injury from the Navy's training exercises, any such injury is outweighed by the public interest and the Navy's interest in effective, realistic training of its sailors," Roberts wrote.

In her dissent, Ginsburg said a Navy environmental assessment had predicted "irreparable" harm to marine mammals.

"In my view, this likely harm . . . cannot be lightly dismissed, even in the face of an alleged risk to the effectiveness of the Navy's 14 training exercises," Ginsburg wrote. "There is no doubt that the training exercises serve critical interests. But those interests do not authorize the Navy to violate a statutory command."