In other words, if Beijing wants to deny U.S. forces access to the theater, U.S. and Japanese commanders should reply in kind. They can deploy submarines along the first island chain to fight in concert with surface forces, detachments of missile-armed land troops, and shore-based tactical aircraft.

"Underway on nuclear power", radioed the skipper of USS Nautilus in 1955, after taking history's first nuclear-powered attack submarine to sea for the first time. Nautilus's maiden cruise left an indelible imprint on the navy. Her success, cheered on by the likes of Admiral Hyman Rickover, the godfather of naval nuclear propulsion, helped encode the supremacy of atomic power in the submarine force's cultural DNA.

Things were never the same after that. America built its last diesel-electric sub, once the state of the art, not long after Nautilus took to the sea. Not since 1990 has the U.S. Navy operated conventionally powered boats. It's been longer than that since they were frontline fighting ships. For a quarter-century, then, it's been all nukes, all the time. No U.S. shipbuilder even constructs diesel boats nowadays.

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That was then. Now may be the time to break up the nuclear monopoly. To wit, imagine permanently forward-deploying a squadron of diesel attack boats, or SSKs, to likely hotspots. Such a force would expand America's silent service, reversing the ongoing slide in numbers of hulls. It would do so at reasonable cost in this age of budgetary stress. A standing East Asia squadron would be close to the action. Likely based in Japan and Guam, it would amplify the U.S.-Japanese fleet's prowess vis-á-vis China's navy and merchant marine. It would empower Washington and Tokyo to deny China access to offshore waters without committing the whole fleet of U.S. nuclear-powered boats to the endeavor. And in the process it would open up new vistas for building and reinforcing alliances.

(This first appeared in 2014.)

Greater numbers, middling cost, a heavier punch in battle. That's a major contribution from such humble craft. U.S. submariners' diesel-propelled past could be, and should be, part of their future.

There's nothing new or especially radical about conventional U.S. subs' prowling the Western Pacific deep. They did so to devastating effect during World War II. For instance, the Philippine Islands was home to the largest concentration of U.S. submarines in the Pacific on the eve of hostilities. U.S. commanders squandered a golden opportunity to run wild against transports carrying Japanese troops to invade the Philippines. But their missed opportunity doesn't detract from subs' potential to confound opponents amid Asia's intricate nautical terrain. It's an exception that proves the rule.

And indeed, American submarines vindicated their potential in ensuing years. U.S. Pacific Fleet boats were among the first vessels to return to Asia following the Japanese raid on Pearl Harbor. Ordered to sea while the battle line was still ablaze at Ford Island, they helped dismember an island empire. Empires like Japan's depend on ships to ferry all manner of warmaking materiel—raw materials, foodstuffs, finished goods—hither and yon. Take away seaborne movement and you cut the lineaments binding the imperial enterprise together.

The submarine campaign grew more and more effective as the U.S. offensives undulated across the Central and South Pacific. U.S. Navy, Marine and Army amphibious forces wrested outer islands from Japan, letting the navy position, maintenance and logistics outposts closer to the foe. Submarine tenders—floating repair and supply depots for all intents and purposes—staged support operations westward of Hawaii. As the transpacific campaigns progressed, boats wasted less time transiting to and from assigned hunting grounds. They spent more time strewing the seafloor with enemy merchantmen and men-of-war.

Forward bases, then, offset the tyranny of distance—allowing the submarine force to mount a stifling presence in Asian waters. Wartime prime minister General Hideki Tōjō catalogued submarine warfare among three critical determinants of Japan's defeat—high praise from someone in a position to know.

And afterward? Ravaged by undersea combat during World War II, Japan built an impressive submarine force of its own to help prosecute the Cold War. Japan Maritime Self-Defense Force (JMSDF) diesel boats turned geography to advantage, lurking in and around the straits that pierce Asia's offshore island chains. Crews monitored and encumbered east-west movement between the China seas and the Western Pacific. Soviet skippers often balked at attempting the passage. The JMSDF, in short, forged itself into a lethal weapon for a cold war beneath the waves.

And so it remains. JMSDF Soryu-class diesel attack boats are the biggest boats of their type, and they're acclaimed among the best—for good reason. Their size lets them carry large amounts of fuel, weaponry and stores, making long patrols feasible. The depths offer a sub its best concealment. Accordingly, Japanese SSKs are outfitted with air-independent propulsion, obviating their need to surface and snorkel frequently. That's an Achilles' heel of older diesel subs. Soryus, then, can remain underwater for long stretches, evading detection from the surface or aloft. And their acoustic properties are excellent while submerged—helping them elude enemy passive sonar. What adversary sonar men can't hear can hurt them.

In short, Soryus are optimized for plying the China seas and Western Pacific. Those are precisely the waters the U.S. Navy, Marine Corps, and Coast Guard singled out as crucial in the 2007 Maritime Strategy, the sea services' most authoritative statement of how they see the strategic environment and intend to manage it. Soryu SSKs are proven platforms manned by experienced mariners who can bequeath their knowledge to their U.S. comrades. That makes these boats a logical common platform around which to build a combined SSK squadron.

Let's evaluate the Soryus' candidacy while remaining mindful that many other capable diesel subs—for instance, the German Type 214—are also on the market and worth considering. In operational terms, what would a U.S.-Japanese force do? Its strategic rationale would be straightforward: it would turn access denial against China, its foremost contemporary practitioner. Reciprocity is a fine thing. Or, if you prefer your strategic wisdom colloquial, paybacks are a b*tch.

In other words, if Beijing wants to deny U.S. forces access to the theater, U.S. and Japanese commanders should reply in kind. They can deploy submarines along the first island chain to fight in concert with surface forces, detachments of missile-armed land troops, and shore-based tactical aircraft. Combined-arms forces could:

-     Keep the People's Liberation Army (PLA) from wresting away beachheads in the island chain. Bursting through the island chain would turn Japan's southern flank (and Taiwan's northern flank), compromising Japanese territory while making it far harder to cordon off the China seas. Defending the islands should be Job One for allied forces.

-     Expel China's flag from crucial seaways. Plugging, say, Miyako Strait (south of Okinawa) with submarines while erecting overlapping fields of anti-ship-missile fire overhead would give the most determined PLA Navy skipper pause. He would think twice before trying to exit the East China Sea for the Western Pacific (or to return to home waters if caught outside).

-     Make transiting north-south along the Asian seaboard perilous in the extreme. SSKs venturing within the island chain could target merchantmen and PLA Navy units with impunity, imposing unbearable costs on Beijing for making trouble.

In short, staging a combined fleet near likely scenes of action would give rise to a kind of mutual assured sea denial. Properly executed, allied anti-access preparations would yield a measure of deterrence vis-á-vis China—enhancing prospects for uneasy peace in the Far East.

But why diesel boats? Isn't the all-nuclear U.S. silent service the world's finest, a silver bullet in the navy's chamber? Yes and no. A U.S. Navy boat remains the odds-on favorite in a duel against any single antagonist. Mass is a severe and worsening problem, however. If the fleet disperses itself all over the map, as global navies are wont to do, it's apt to find itself outmatched at some trouble spot or another. Never mind how capable an individual platform may be. If commanders concentrate assets in one trouble spot, on the other hand, other priorities may go uncovered. Now as ever, quantity has a quality all its own. And quantity is precisely where trouble lies.

The reason for dwindling fleet totals should astound no one. It's dollars and cents. As Cold War-era Los Angeles-class nuclear attack subs (SSNs) retire, they're being replaced not on a one-for-one basis but by fewer, more expensive Virginia-class SSNs. As costs rise and shipbuilding budgets stagnate—if that—downward pressure on numbers mounts inexorably. The fleet is projected to sag from 55 SSNs today to as low as 42 around 2030.

Forty-two sounds like a lot, doesn't it? But naval leaders are forever reminding us that seven-tenths of the earth's surface is covered by water. The seven seas adds up to an awful lot of waterspace for 42 boats to police—especially since a sizable fraction of that tally is in overhaul, routine maintenance, or workups on any given day. Many units, that is, are unavailable for combat duty no matter how sorely they're needed. Shave a third off the raw number of ships and you have a good guesstimate about the number of subs available to some degree or another.