Submarines Must Learn to Play Zone Defense

“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.

On the other side of the ball, a superstar pass defender bolsters the prospects of zone defense while making man-to-man defense a viable option. Indeed, man coverage by savvy, dexterous defenders represents the tactic of choice, in football as in undersea warfare. Nevertheless, zone defense can prove effective under certain circumstances. To choose, winning coaches get to know themselves and the adversary—just as successful naval commanders know and exploit their forces’ capabilities, compensate for their weaknesses and offset an antagonist’s strengths while exploiting its weaknesses. They tap their competitive advantages.

It’s possible to take zone defense too far. The prevent is a risk-averse, deeply conservative variant of zone defense. It’s an expedient for slowing down an opponent who’s advancing on a broad front, isn’t far behind on the scoreboard and is working on a tight deadline—usually the game’s end. The maximum number of defenders—six, making this a “Cover 6” scheme—drop into deep coverage to frustrate the long pass. They leave the short pass open as a result, yielding yardage for the sake of foreclosing the big play that could lose them the game. Prevent defense is about running out the clock without giving up a fatal big play.

While the naval balance may force the U.S. Navy into a zone defense of some type, then, prevent defense is one to avoid. Think about the differences between football and combat. Undersea warfare doesn’t unfold on the clock, for one thing. Rather than go uptempo, an enemy fleet can take its time, probing the defense for weak spots to slip through. It may and likely will unearth seams, considering the vast volumes of water defenders must monitor to prevail in open-ocean antisubmarine warfare (ASW). Deliberately surrendering waterspace to mount a defense-in-depth loosens up the defensive cordon, potentially letting the quarry escape into the briny deep—to be discovered again only when friendly ships start exploding and descending to Davy Jones’s locker.

Hence Breedlove’s unease at playing zone defense beneath the waves. He’s right to find the prospect disquieting. Strategic theorist Carl von Clausewitz holds forth on the hazards of dispersing combat power: “there is no higher and simpler law of strategy than that of keeping one’s forces concentrated. No force should ever be detached from the main body unless the need is definite and urgent” (Carl’s emphasis). The more a zone defense disperses assets, the leakier the defense, and the greater the likelihood a silent-running foe will sneak past the sentinels. U.S. commanders must bear this Clausewitzian wisdom in mind when designing and prosecuting defensive schemes.

Now, a zone defense patterned on basketball may be worth exploring. Properly executed, it would alleviate some of the problems over which Clausewitz frets. Such an approach demands that commanders turn the tactical setting to American advantage. And indeed, Julian Corbett—a fervent acolyte of Clausewitz—proposes a strategy that mimics the basketball more than the football approach. Corbett urges commanders to choose their terrain wisely, then disperse the fleet as widely as possible so long as it can still concentrate fighting strength at the decisive point after detecting an enemy boat.

The fleet’s disposition, writes Corbett, should be “elastic enough to permit it to cover a wide field without sacrificing the mutual support of its parts.” “Elastic cohesion” is the watchword for his brand of zone defense.

How to incorporate elasticity into U.S. submarine operations, balancing between concentrating power and watching wide sea areas? If commanders can arrange matters so that red-team subs must pass through a strait or other narrow passage to reach their patrol grounds—say, if the U.S. force chooses to defend the approaches to a strait—then something evoking a 2–3 zone might work. That’s a defense that assigns each of the five team members a sector around the basket to guard. The defenders run to the ball, shifting around within their sectors to render mutual support as the offensive team passes the ball around the perimeter. In other words, a zone defense concentrates defensive strength at shifting points along the periphery to deny the offense the close-in, easy, high-percentage buckets that coaches covet.

A 2–3 defense, then, is like a wall that that reinforces itself wherever the offense tries to penetrate for a layup or short jump shot. Getting inside that protective shell is tough. An effective zone compels the opponent to fire away from long range, where the percentage of successful shots dwindles markedly. The 2–3 zone represents a winning strategy against an opponent that lacks long-range sharpshooters, or an uptempo offense that lets the offense shoot more often—offsetting the lower percentage of shots that go in.

So concentration and dispersal of power are critical. Unlike coverage shells in football—the closest thing to zone defense on the open sea—a 2–3 defense masses the defenders around something the foe values rather than dispersing them in hopes of catching a hostile force advancing along a broad, indistinct front. That’s a more promising defensive philosophy than the NFL variant—and something silent-service tacticians should explore. A 2–3 zone would constitute the passive component, providing for close-in defense. In the meantime a superstar free safety—a highly capable sub—could prowl the periphery, harassing oncoming red-team members. That’s the active component.

Combining such sports-inspired measures would let U.S. submariners erect a stout defense—call it a hybrid zone/man, active/passive defense. Making geography an ally, then, promises major dividends. It bolsters prospects for success with even a meager number of subs. Study the map when devising maritime strategy.

So much for executing an ambitious strategy on a shoestring. How can a sports-minded U.S. Navy boost its future effectiveness in undersea warfare? First of all, the navy—like any old ball coach—must constantly upgrade the team roster, recruiting more and better players. And there’s no upper limit on the number of players except budgets. The rules governing sports are somewhat artificial in that they cap the number of players on the roster, and on the field or court. Not so in maritime operations. There’s no referee to enforce rules of high-seas combat. Each contender brings as many assets to the contest as it can afford and is willing to spare for an endeavor.

Numbers matter, then. Quality—well-made hulls, sensors and armaments—is indispensable. But quantity has a quality all its own. Sea power is a conscious political choice. America must make the conscious political choice to build more SSNs, giving the navy sufficient assets to prosecute a globe-straddling strategy.

Second, augmenting the silent service’s striking power will impart new effectiveness to U.S. defenses, whether zone or man-to-man. American SSNs are the best in the world on a boat-for-boat basis, but their striking reach is woefully short, forcing them to remain close to one another in a defensive screen or while tailing an opponent. An SSN can accomplish little unless it closes to within ten nautical miles of a target, the extreme firing range of its torpedoes. That’s too short a reach. Fielding longer-range ASW implements is a must, as submarine-force potentates acknowledge.

Clausewitz describes the best defense not as “a simple shield”—a purely passive defense—but as “a shield made up of well-directed blows.” ASW weapons boasting ranges from, say, one to two hundred nautical miles would let the silent service create overlapping fields of fire from a distance, expanding the fleet’s geographic coverage. The long arm of U.S. SSNs would render zone defense more palatable than it is today while buttressing man defense.

Third, officialdom should augment the subsurface contingent through alliances and low-cost subs. Look at the Far East, where the PLAN and its sister services are making noise along Asia’s first island chain. Why not buy Japanese to offset Chinese pretensions? The U.S. Navy could purchase a squadron of Japanese Soryu-class diesel-electric attack boats (SSKs) at low cost, merge them into a combined U.S.-Japanese undersea fleet and station them permanently along the Japanese archipelago.

You don’t need a nuclear-propelled boat, with its unlimited cruising range and long at-sea endurance, to stage a zone defense in, say, the straits piercing the Ryukyu island chain. You can do it with an SSK—and you can buy four or five SSKs for the cost of a single Virginia-class SSN, the U.S. Navy’s latest. Substituting diesel for nuclear boats would free up pricey SSNs for duty elsewhere on the map. Plus, how’s a combined fleet for a token of solidarity with America’s foremost Asian ally?

And fourth, gee-whiz technology is creating new types of players to join the roster. It’s a truism among bubbleheads that the best antisubmarine platform is another submarine. Yet ASW has never been solely a sub-on-sub affair. Surface warships have battled subs for over a century now, supplemented by combat aircraft as aviation technology matured. Unmanned vessels and remote sensors can help plug the gaps in a zone defense, tightening up the 2–3 zone in confined seas while making open-ocean ASW a more realistic prospect—despite the colossal expanses of water involved.

For instance, “subullites,” or sonar nodes scattered around the deep seafloor, will gaze upward across a wide field of view, listening for passing subs. Once cued in, unmanned surface and subsurface craft will trail hostile boats for long stretches of time, vectoring SSNs or other ASW assets to the scene should a fight ensue. Buying such high-tech, low-cost assets in bulk only makes sense. Not only will such assets firm up zone defenses, they will return some of the active, man-to-man character to silent-service operations—and restore the efficacy of U.S. maritime strategy beneath the waves.

See, General Breedlove? Sports metaphors do reveal wisdom about martial enterprises. Run silent, run deep, my friends!

James Holmes is Professor of Strategy at the Naval War College and coauthor of Red Star over the Pacific. The views voiced here are his alone.

Image: Army guard Cleveland Richard looks to pass during the Black Knights 76-71 victory over Navy on Jan. 25, 2009 at Christl Arena. Flickr/West Point