In spite of a presidential waiver, last week US District Judge Florence-Marie Cooper re-issued a January injunction against the Navy's use of active sonar in anti-sub training off southern California, claiming it violates existing environmental statutes.

Why? It might harm whales and dolphins.

OK, everyone loves Flipper and Shamu. But our sailors need this critical training before heading out on extended (six- to nine-month) deployments to East Asia and the Middle East, where they could come up against potential adversaries' submarines.

The ability to detect enemy submarines is essential to our national defense, and active sonar is the only existing technology that can track modern, deadly quiet diesel-electric and fuel-cell submarines.

The sub threat is alive and well. Some 41 countries operate more than 400 submarines worldwide, including potential adversaries such as China, Iran and North Korea. And diesel subs (which are super stealthy and relatively cheap) are an important platform for countries seeking an asymmetric counterpunch to superior naval forces like ours - still the world's most powerful.

For example, both China and Iran operate the Kilo-class diesel sub, a $150 million boat that can use a $300,000 Russian torpedo to sink a state-of-the-art $1.5 billion US destroyer. And Chinese diesel submarines have made a nasty habit of shadowing US battle groups in the Western Pacific lately - including popping up within weapon's range of a carrier, the USS Kitty Hawk, during an exercise last fall.

Should America and Iran come to blows, the Iranians would almost assuredly use both the patrol-boat-swarm tactics we saw recently and deploy their Kilo-class subs against US naval forces in the vicinity of the Persian Gulf.

North Korea doesn't have Kilos, but it does use diesel and mini-subs for intelligence collection and landing spies in South Korea and Japan. In wartime, these boats would insert special-operations troops (North Korea has the world's largest spec-ops forces) behind US-South Korean lines.

The area off southern California - the subject of the injunction - is the best place for Navy carrier battle groups or individual ships in the Pacific to get anti-submarine warfare training before deploying.

The Navy operates in many areas characterized by shallow waters, deep depths, underwater ridges and challenging acoustics similar to parts of the Southern California Operating Area - making it irreplaceable for ASW training.

In fact, the Catalina Basin uniquely replicates conditions the Navy might face in such potential hot spots as the waters of the Strait of Hormuz off Iran or the Taiwan Strait between China and Taiwan.

The Navy has been taking its own steps to protect sea life, too. Its commanders already follow 29 "mitigation measures" - that is, safeguards - to protect marine mammals when they're using mid-frequency active sonar during ASW exercises.

The Navy not only reduces or cuts power to the sonar if marine mammals are known to be present, it also takes measures such as posting lookouts with night-vision devices to watch for sea life.

It's also been granting universities millions every year - $18 million is planned for 2008 - for research into how sonar affects the marine environment.

Moreover, the Navy isn't the worst offender. Statistically, the number of marine mammal "strandings" (like beached whales) owing to sonar is low compared to those caused by commercial fishing (the biggest problem) or Mother Nature.

No one wants to needlessly damage sea life. But it's crucial the Navy continue this realistic active-sonar training to protect our national interests - including our sailors - from the growing threat of quiet diesel-electric subs.

The National Environmental Policy Act expressly lets the president temporarily exempt military exercises from its green constraints, which he did. But now the Navy needs to convince a superior court that preparing our ships and crews for deployment is an urgent matter for our national defense.

The courts need to look well beyond the local California coastal waters to see the serious national-security challenges our Navy faces overseas - and resist interfering with the vital training our brave sailors need.

Peter Brookes is a Heritage Foundation senior fellow and former US deputy assistant secretary of defense.