“Zone defense”? In submarine warfare? Fuhgeddaboudit: that’s basketball and football stuff, not maritime strategy. And yet the sports metaphor—the handiwork of U.S. Air Force Gen. Philip Breedlove—instructs. It suggests the U.S. Navy needs to bulk up its fleet of nuclear-powered attack submarines (SSNs). Otherwise a growing mismatch between the demand for and supply of SSNs may force the “silent service” into suboptimal tactics, degrading the navy’s mastery of the seas and compromising America’s strategic position in important theaters. That would be a Bad Thing.

Last February, General Breedlove, then the overseer of the U.S. European Command (EUCOM), sketched a somber picture of the undersea naval balance. U.S. House Armed Services Committee members inquired whether the navy could furnish enough attack submarines to meet EUCOM’s demand for them. His unequivocal reply: no. Increasingly sophisticated and numerous competitors—navies on the make, notably the Russian Navy and China’s People’s Liberation Army Navy (PLAN)—have amplified the demand for U.S. Navy SSNs around the western European periphery. As the number of prospective foes surges, the demand for attack boats has come to outstrip the supply.

No longer, that is, can the navy allocate an SSN to trail every red-team boat making its way into the open sea, the way it did during the Cold War. Adds Breedlove’s U.S. Pacific Command counterpart, Adm. Harry Harris, the Indo-Pacific theater “suffers from a shortage of submarines today,” owing both to resurgent Russian naval ambitions and to a China making its seaward turn. “My requirements are not being met,” declares Harris. Seas washing against the East and South Asian rimlands are going partly unpoliced—impairing America’s posture in these all-important marginal seas.

Are such complaints just hype? The navy’s official requirement for SSNs is forty-eight, but fully fifty-three are in service at present. The silent service boasts resources in abundance—five extra hulls!

Trouble is, the navy set the forty-eight-boat requirement a decade ago, when the enemy below was far less troublesome than today. Modest threat, modest fleet. The PLAN subsurface fleet was building but far from complete. The intricacies of naval nuclear propulsion were still vexing Chinese shipwrights. Russia’s return to the high seas remained a gleam in Vladimir Putin’s eye. How to dismantle Soviet-era subs posed a bigger concern than laying the keels for new ones.

In short, forty-eight SSNs seemed like a reasonable tally a decade ago. Today, not so much. The undersea threat is worsening at the same time the United States’ SSN fleet is declining in brute numerical terms.

Finances are the chief culprit. The navy is struggling to sustain adequate numbers, while at the same time retiring Cold War–era Los Angeles–class boats and replacing Cold War–era Ohio-class nuclear-powered ballistic-missile subs. The Ohio replacement project is so expensive that it alone threatens to consume the United States’ entire shipbuilding budget during the 2020s. In short, the differential equation is working against the U.S. Navy. SSNs are retiring faster than they can be replaced. Inexorable budgetary math may drive the SSN fleet as low as forty-one boats by the 2030s—below the figure for the relatively friendly seas of a decade ago—even as demand for SSNs waxes.

To balance the equation, the sea-service leadership will probably boost its requirement for SSNs. The service recently launched a series of studies widely expected to espouse bulking up the U.S. Navy fleet as a whole—including its subsurface component. In the meantime, as Breedlove forecasts, the shortage of attack boats will compel the silent service to play “zone defense” rather than “man-to-man.” Otherwise, it stands little chance of keeping pace with increasingly contested oceanic surroundings.

In short, circumstances—more formidable prospective foes coupled with scanty resources and competing demands—are forcing American submariners to depart from time-tested operational practices. Breedlove apologized for deploying a sports metaphor. “I hate to simplify this,” he told Congress, “but it’s just a very simple way of understanding.” That’s unduly humble. Zone and man-to-man defense may be inexact analogies for undersea combat, but they’re close enough to fuel thought and operational entrepreneurship. You take your insight where you find it.

As Breedlove observes, submariners, affectionately known as “bubbleheads,” have long preferred to play a version of man-to-man defense. The logic behind the one-on-one approach is straightforward. Sea-power sage Julian S. Corbett observes that you only know a ship’s position at three points—at most—during its voyage. You find a sub at one of those points and follow it, keeping tabs on its whereabouts.

With good intelligence, first, you may learn a foe’s seaport of origin and can lurk offshore to make contact. You may know its destination, secondly, assuming it’s bound for a fixed point on the nautical chart, and assuming its commander doesn’t receive new orders changing the mission in midcourse. And third, says Corbett, ships congregate at “focal areas” to pass by land or some underwater obstacle. Shipping lanes converge, for instance, at straits like Malacca, Hormuz and Gibraltar. These are quintessential focal areas, good places to find the red team in transit.

American bubbleheads acted on Corbettian logic during the Cold War. They excelled at detecting Soviet attack and ballistic-missile boats at key junctures in their journeys. Attack boats tailed their antagonists, acquainting themselves with Soviet hardware and operating practices while granting senior U.S. commanders the option of assailing the Soviet fleet early should war ensue. Man-to-man defense constitutes a more reliable method for locating and targeting foes, compared to zone defense. If you know an enemy’s position, you stand a better chance of latching onto him—and staying latched.

Man defense, in short, constitutes an intensely active, offensive-minded approach to subsurface operations. But it does require numbers, one SSN per adversary boat. A zone defense, by contrast, would assign each SSN a patrol sector and count on sonarmen equipped with high-tech sensors to detect enemies passing through the sector. That’s a more dubious proposition. Subs operate in three dimensions. Even a modest-sized sector, consequently, encompasses a massive volume of water which may—or may not—contain a hostile boat.

Uncertainty is the hazard of the passive approach. It’s hard enough to find a sub when you know one’s there. It’s doubly hard to remain alert for long stretches when a sub may, or may not, be nearby. Basic physics also works on behalf of a “hider” trying to elude the “finder.” Large bodies of water diffuse, refract and attenuate sound and light—indeed, electromagnetic emissions of all types. Changes in temperature, pressure and salinity bend sound waves, playing tricks on sonar operators, the most reliable sub hunters today. Subs can hide beneath or above layers in the water, disguising their presence even from nearby foes.

Bottom line, it’s easier for the red team to slip through a zone defense undetected than to shake off tenacious man-to-man defenders. The probability of an intercept—and, in battle, a kill—diminishes as a fleet resorts to passive measures.

Now, this is not a counsel of despair. There’s more than one way to run a zone defense, depending on the turf being protected. The more U.S. zone-defense tactics come to resemble a “prevent” defense in football, in which dispersed defenders try to guard a massive swathe of real estate, the less effective they’ll be. The more U.S. tactics resemble a “2–3 zone” in basketball—a zone in which defenders guard a compact piece of real estate surrounding something important to the foe—the better off the silent service will be. Commanders should compete in settings that work in their favor.

Like naval commanders, NFL and NCAA coaches try to protect a defensive perimeter—the line of scrimmage—while hedging should a running back break through the defensive perimeter, or should the quarterback heave the ball over the line to a receiver downfield. To guard against the pass, they deploy what football insiders term “coverage shells.” Coverage shells divert defenders from the line of scrimmage, assigning each a sector to defend downfield. The defender’s goal: to break up or intercept passes that come into his sector. Coverage shells amount to a defense-in-depth, but this approach comes at a price. The tradeoff is that keeping defenders back to foil passes softens the defense near the line of scrimmage. Opportunities for short passes open up underneath. A team can march down the field using incremental yardage gains rather than stake everything on the long bomb.

Quite a quandary. How do coaches decide which defensive scheme to run? Well, there’s geography. The field is a rectangle, in effect a featureless plain. But the ball can be snapped closer to one sideline, compressing the offense and defense on that side of the field while stretching them on the other. And as the offense closes on the opponent’s end zone, its progress compacts the amount of ground the defense must protect—making it harder to either run or throw for additional yardage. Physical space molds strategy and tactics, whether on a flat rectangle or when hunting subs in the depths.

And then there’s the all-important human factor. Coaches gauge the individual talents of players, both the opponents’ and their own. When facing off against an opponent without a good QB, it makes sense to stack the defense close to the line of scrimmage. If the offense has no air attack and you take away the ground game, the offense grinds to a halt. If the opponent who has a Tom Brady, a QB who can pass with scalpel-like precision but isn’t much of a running threat, a softer defense up front coupled with more robust zone defense may be in order. And if the opponent has a Cam Newton who can fling the ball or run it himself, you’re in real trouble. You’d better recruit defenders of like caliber to blunt that dual threat.