Held: The preliminary injunction is vacated to the extent challenged by the Navy. The balance of equities and the public interest--which were barely addressed by the District Court--tip strongly in favor of the Navy. The Navy's need to conduct realistic training with active sonar to respond to the threat posed by enemy submarines plainly outweighs the interests advanced by the plaintiffs. Pp. 10-24.

     (a) The lower courts held that when a plaintiff demonstrates a strong likelihood of success on the merits, a preliminary injunction may be entered based only on a "possibility" of irreparable harm. The "possibility" standard is too lenient. This Court's frequently reiterated standard requires plaintiffs seeking preliminary relief to demonstrate that irreparable injury is likely in the absence of an injunction.

     Even if plaintiffs have demonstrated a likelihood of irreparable injury, such injury is outweighed by the public interest and the Navy's interest in effective, realistic training of its sailors. For the same reason, it is unnecessary to address the lower courts' holding that plaintiffs have established a likelihood of success on the merits. Pp. 10-14.

     (b) A preliminary injunction is an extraordinary remedy never awarded as of right. In each case, courts must balance the competing claims of injury and consider the effect of granting or withholding the requested relief, paying particular regard to the public consequences. Weinberger v. Romero-Barcelo, 456 U. S. 305, 312. Military interests do not always trump other considerations, and the Court has not held that they do, but courts must give deference to the professional judgment of military authorities concerning the relative importance of a particular military interest. Goldman v. Weinberger, 475 U. S. 503, 507.

     Here, the record contains declarations from some of the Navy's most senior officers, all of whom underscored the threat posed by enemy submarines and the need for extensive sonar training to counter this threat. Those officers emphasized that realistic training cannot be accomplished under the two challenged restrictions imposed by the District Court--the 2,200-yard shutdown zone and the power-down requirement during surface ducting conditions. The use of MFA sonar under realistic conditions during training exercises is clearly of the utmost importance to the Navy and the Nation. The Court does not question the importance of plaintiffs' ecological, scientific, and recreational interests, but it concludes that the balance of equities and consideration of the overall public interest tip strongly in favor of the Navy. The determination of where the public interest lies in this case does not strike the Court as a close question. Pp. 14-16.

     (c) The lower courts' justifications for entering the preliminary injunction are not persuasive. Pp. 16-21.

          (1) The District Court did not give serious consideration to the balance of equities and the public interest. The Court of Appeals did consider these factors and conclude that the Navy's concerns about the preliminary injunction were "speculative." But that is almost always the case when a plaintiff seeks injunctive relief to alter a defendant's conduct. The lower courts failed properly to defer to senior Navy officers' specific, predictive judgments about how the preliminary injunction would reduce the effectiveness of the Navy's SOCAL training exercises. Pp. 16-17.

          (2) The District Court abused its discretion by requiring the Navy to shut down MFA sonar when a marine mammal is spotted within 2,200 yards of a sonar-emitting vessel. The Court of Appeals concluded that the zone would not be overly burdensome because marine mammal sightings during training exercises are relatively rare. But regardless of the frequency of such sightings, the injunction will increase the radius of the shutdown zone from 200 to 2,200 yards, which expands its surface area by a factor of over 100. Moreover, because training scenarios can take several days to develop, each additional shutdown can result in the loss of several days' worth of training. The Court of Appeals also concluded that the shutdown zone would not be overly burdensome because the Navy had shut down MFA sonar several times during prior exercises when marine mammals were spotted well beyond the Navy's self-imposed 200-yard zone. But the court ignored undisputed evidence that these voluntary shutdowns only occurred during tactically insignificant times. Pp. 18-20.

          (3) The District Court also abused its discretion by requiring the Navy to power down MFA sonar by 6 decibels during significant surface ducting conditions. When surface ducting occurs, active sonar becomes more useful near the surface, but less effective at greater depths. Diesel-electric submariners are trained to take advantage of these distortions to avoid being detected by sonar. The Court of Appeals concluded that the power-down requirement was reasonable because surface ducting occurs relatively rarely, and the Navy has previously certified strike groups that did not train under such conditions. This reasoning is backwards. Given that surface ducting is both rare and unpredictable, it is especially important for the Navy to be able to train under these conditions when they occur. Pp. 20-21.

          (4) The Navy has previously taken voluntary measures to address concerns about marine mammals, and has chosen not to challenge four other restrictions imposed by the District Court in this case. But that hardly means that other, more intrusive restrictions pose no threat to preparedness for war. The Court of Appeals noted that the Navy could return to the District Court to seek modification of the preliminary injunction if it actually resulted in an inability to train. The Navy is not required to wait until it is unable to train sufficient forces for national defense before seeking dissolution of the preliminary injunction. By then it may be too late. P. 21.

     (d) This Court does not address the underlying merits of plaintiffs' claims, but the foregoing analysis makes clear that it would also be an abuse of discretion to enter a permanent injunction along the same lines as the preliminary injunction. Plaintiffs' ultimate legal claim is that the Navy must prepare an EIS, not that it must cease sonar training. There is accordingly no basis for enjoining such training pending preparation of an EIS--if one is determined to be required--when doing so is credibly alleged to pose a serious threat to national security. There are many other remedial tools available, including declaratory relief or an injunction specifically tailored to preparation of an EIS, that do not carry such dire consequences. Pp. 21-23.

518 F. 3d 658, reversed; preliminary injunction vacated in part.

     Roberts, C. J., delivered the opinion of the Court, in which Scalia, Kennedy, Thomas, and Alito, JJ., joined. Breyer, J., filed an opinion concurring in part and dissenting in part, in which Stevens, J., joined as to Part I. Ginsburg, J., filed a dissenting opinion, in which Souter, J., joined.[/quote]