Chief Justice Roberts wrote in his opinion, "the most serious possible injury would be harm to an unknown number of marine mammals that they study and observe".

[3] The U.S. Navy had scheduled 14 training exercises through January 2009 off the coast of Southern California involving the use of "mid-frequency active sonar" to detect enemy submarines.

Whales have been found beached in Greece, the Canary Islands, and in the Bahamas after sonar was used in the area, and necropsies showed signs of internal bleeding near the ears.

However, the Court continued, even if plaintiffs had shown irreparable injury (and, too, likelihood of success on the merits), it is "plainly outweighed" by the Navy's interest in effective, realistic training of its sailors.

[10] A report published in 2009 by the Congressional Research Service noted that the Supreme Court accepted the case "as a challenge to a preliminary injunction, rather than to the merits of petitioners' statutory claims".