The Supreme Court seemed closely split on whether President George W. Bush can exempt the Navy from federal environmental laws, a case pitting protection of whales against military training exercises.
In the most significant environmental case of its new term, the court is reviewing a ruling that required the Navy to take various precautions to minimise harm to dozens of species of whales and dolphins.
The four liberal justices expressed concern over the administration's failure to do an environmental impact statement before sonar training exercises began off the southern California coast.
Environmentalists say the intense sound waves used in sonar training exercises can harm or even kill endangered whales, possibly by interfering with the marine mammals' dive patterns.
During the arguments, the conservative justices appeared supportive of the administration's argument that judges should defer to the judgment of the Navy and Mr Bush, and allow the submarine-hunting exercises.
After a judge issued a preliminary injunction imposing numerous restrictions on the Navy, Mr Bush intervened. He cited the national security necessity of the training and exempted the Navy from the environmental laws at the heart of the legal challenge.
A US appeals court rejected the White House's effort to exempt the Navy from the laws, prompting the administration to appeal to the Supreme Court.
Solicitor General Gregory Garre, the administration's top courtroom lawyer, told the justices the Navy's ability to use sonar to locate and track enemy submarines is "vitally important" and "critical to the nation's own security."
Liberal Justices David Souter and Ruth Bader Ginsburg asked why the administration planned to complete an environmental impact statement in January, rather than doing it in February of 2007 when the exercises began.
Chief Justice John Roberts seemed concerned that the judge had not properly balanced the harm to marine mammals with the "substantial challenge" imposed on the Navy.
A ruling in the case is expected early next year.