Here's What You Need to Know: If operating two Queen Elizabeth ships and their stealth aircraft proves pricier than forecast—hardly an unlikely prospect—the carriers themselves might come under scrutiny. Having reduced the Royal Navy to a couple of fleet units to save money, lawmakers might deploy the same bean-counting logic to justify dismantling the fleet units themselves. Allied effectiveness would suffer.

Talk about role reversal. A long century ago, starting in 1909, Great Britain entreated its Pacific dominions—Canada, New Zealand, Australia—to construct “fleet units” to supplement a Royal Navy that confronted multiple challengers in multiple theaters. A fleet unit was a modular task force composed of a cruiser and its coterie of destroyers. Naval potentates such as Adm. Jacky Fisher expected each dominion to construct and maintain one. It would serve as the national navy while doubling as a module in an imperial navy. In peacetime each fleet unit could perform routine functions on its own, acting as a standalone armada. Or dominion navies could merge into a grand Pacific fleet alongside Royal Navy forces when storm clouds gathered. Having massed for action, the imperial fleet would face down some predator—presumably the Imperial Japanese Navy, a force casting covetous eyes on maritime Asia.

The fleet unit, then, was a strategic concept for a mass-production age. Fleet units comprise building blocks of a multinational navy. In theory they’re interchangeable, featuring standard hardware, training and doctrine. A common language, cultural heritage and worldview helps. Plug and play!

Assembling an interoperable imperial navy while conscripting dominion governments to help bear the burden represented Britain’s way of regenerating its naval posture in the Pacific Ocean. In those days Europe was choosing up sides for the Great War. The Kaiser’s Germany was bolting together a battleship fleet hard by the British Isles, and menacing British shores with each keel laid. The German High Seas Fleet taking shape in the North Sea thus beckoned Royal Navy ships homeward from distant stations, there to run a naval arms race in defense of the homeland. Evacuating Britain’s Pacific holdings to compete with Germany left a power vacuum, tempting an Imperial Japan flush with high-seas victories over Russia and China. The fleet-unit concept purported to fill that vacuum—applying counterpressure to offset Japanese ambitions.

Today, though not in so many words—and, in fact, perhaps with little deliberate strategic design—Britain is reconfiguring its Royal Navy as a fleet unit for service in an imperial fleet. America is the liberal imperium now under strain. It’s in need of material aid in the Far East to offset another would-be hegemon, namely China.

That seems an unlikely role for a Royal Navy in decline. London is reportedly set to divest the Royal Navy of its “gator” fleet, the amphibious transports and support vessels that land Royal Marines on foreign shores. Much of a navy’s capacity to exploit control of the sea resides in amphibious ships and marines—the very forces now on the chopping block. In effect top political leaders are demoting amphibian power projection to an afterthought in British strategy. Others will project power onto foreign shores after the Royal Navy helps win fleet battles. No longer will Great Britain possess the wherewithal to conduct maritime campaigns as an independent great power. It is relegating itself to supporting-actor status.

Decommissioning the amphibious contingent will leave behind a specialist surface navy centered on two Queen Elizabeth-class supercarriers now preparing to join the fleet. The Royal Navy is morphing from a balanced fleet into something less.

We can consult a Briton for insight into these matters. Crudely speaking, Britain’s homegrown sea-power theorist, Julian S. Corbett, partitions naval warfare into two phases. Two fleets fight for “command” of important waterways. Command of the sea means driving off or sinking rival fleets, and thereby creating a safe nautical sanctuary from which to wage war at sea, on land, or aloft. Antagonists try to deny command to each other while wresting it away for themselves.

And afterward? Success entitles the victor to exercise command of waters scoured of enemy forces. Command represents an enabler for such workmanlike missions as policing the sea, raiding enemy merchantmen, pummeling targets on foreign shores, or landing forces on dry ground to project power inland. These are the dividends of maritime command. Seamen think in terms of garnering glory on the high seas, yet boots slogging across muddy battlegrounds are what win wars. And depositing troops, equipment, and stores on land in bulk demands amphibious transports—ship types whose days appear numbered in Great Britain.

In that sense the Royal Navy appears destined to become a partial navy: it will excel at fleet-on-fleet combat while boasting minimal capacity to exploit the gains from fleet combat. The revamped Royal Navy will fall short after the fight.

Why the turnabout in British maritime strategy? It owes to a mix of financial constraints and strategic considerations, but cost-cutting seems to predominate. It’s been decades since Britannia ruled the waves. Having gotten by for so long without a globe-spanning navy, it seems British leaders have concluded they no longer need to fund a balanced fleet—in other words, a force equipped and trained to execute missions throughout the continuum from peacetime to wartime missions.

To shed costs, accordingly, Parliament and Prime Minister Theresa May’s government are consciously unbalancing the Royal Navy. Rather than disperse finite resources in an effort to maintain ever-dwindling numbers of all ship types, they are concentrating resources on a few specialist capabilities—chiefly those comprising carrier aviation forces—alongside traditional strengths in undersea warfare and mine countermeasures. They’re refocusing Britain’s navy on functions associated with battling for and holding command of the sea, while soft-pedalling capabilities associated with exercising nautical command.

British seafarers, in other words, will concern themselves mainly with blue-water operations, mainly with fleet-on-fleet duels, and mainly with the early stages of marine conflict.

Now, this is largely unobjectionable from my parochial ‘Mercan standpoint. The United States is struggling to “rebalance” enough naval forces to the Pacific to offset China’s swelling military might. The U.S. Navy could use the help in sea fights, whereas it can probably satisfice with the gator fleet it fields.

The Royal Navy could render direct aid, returning to regional seaways decades after withdrawing from east of Suez. Once there, British task forces could help shore up deterrence vis-à-vis China. Three U.S. Navy carrier strike groups just operated together in Northeast Asia. In the future a Royal Navy flattop might join in—helping constitute a formidable striking arm while telegraphing a powerful message about allied solidarity.

Or, more plausibly, allied leaders could draw up a geographic division of labor. British seamen could assume more responsibility for maritime defense in the Atlantic theater, making sure the Atlantic remains tranquil while American carrier forces ply Far Eastern waters. In the process the Royal Navy would free up U.S. forces for East Asia.

Either way, fleet-unit logic remains as compelling today as it was a century ago. Direct or indirect, British help with managing high-seas affairs would prove invaluable to the common cause. True to the fleet-unit approach, the Royal Navy should shape naval forces that are as interoperable with their U.S. Navy counterparts as possible. An aircraft-carrier flotilla able to plug-and-play into a U.S.-led carrier task force, or even take command of such a task force, would be ideal. That the Queen Elizabeth carriers will deploy air wings centered on the same aircraft as U.S. Navy nuclear-powered flattops—F-35 Lightning II stealth fighters—augurs well for combined naval operations.

The advent of a modular Royal Navy, then, represents a good-news, bad-news story for those of us on this side of the Atlantic. A carrier-centric navy can contend for mastery of the sea, the first and foremost mission for any sea service. Its specialty, open-seas combat, is something the U.S. Navy and other allied forces can sorely use. Presumably, though, the Royal Navy will delegate the job of projecting power ashore to allies. Dismantling the Royal Navy’s gator fleet will swivel British maritime strategy away from land toward managing events on the high seas—and in the process foreclose strategic options for Britain as a freestanding great power.

Thus London will need allied support to prosecute any major seaborne campaign. If no help is forthcoming, Britain might forfeit worthwhile goals. Or Britain’s allies might join an enterprise solely to preserve alliance relations—and thus with little fervor for the enterprise. Popular disillusionment with the transatlantic special relationship could result. Such are the quandaries of entangling alliances—which is why America’s founders warned against them. Seemingly technical questions about navy budgets and fleet design in Great Britain thus raise larger questions about the health and longevity of transatlantic ties.

Denuding the Royal Navy of its capacity for amphibious warfare, then, could give rise to self-defeating entanglements. If London saw the need to act in, say, North Africa yet lacked the capacity to land marines, that might tempt Washington to join in—lending U.S. Navy and Marine Corps capabilities to a venture that might not suit American interests, or that might siphon off resources from U.S. sea services that are already scattered across the seven seas for manifold purposes. U.S. leaders must exercise some self-discipline to avoid further overextending the services.