By Peter Apps, Political Risk Correspondent
WASHINGTON (Reuters) - China has showcased its first aircraft carrier landings while maintenance woes have reduced the United States to a single carrier in the Gulf, pointing to the beginnings of a subtle shift in the balance of naval power.
With South China Sea tensions growing, the threat of Middle East conflict still very real and counterterrorism and counter piracy operations also demanding resources, demands on Western navies - and the U.S. in particular - seem ever-growing.
Even as it emerged that problems with the USS Nimitz would leave Washington unable to maintain its standard two-carrier Gulf force for the first time since 2010, its navy found itself sending new forces to a volatilce eastern Mediterranean.
Tough choices loom, with the U.S. military facing years of tighter spending - and the prospect of even starker reductions from sequestration still very real just as European allies seem less able than ever to offer support.
"None of these developments is overwhelming or shocking in its own right," says Nikolas Gvosdev, professor of national security studies at the U.S. Naval War College.
"But they point to a larger trend. The U.S. is going to have to get used to not always having the capability to be everywhere. There are going to be more gaps, and there are going to be other countries that fill those gaps."
With its own domestic energy production potentially freeing the United States from dependence on Middle East oil, some are beginning to ask whether the world's pre-eminent superpower should bear the cost of being global maritime policeman everywhere.
"I don't believe you're going to see the U.S. pull back from being the only force capable of protecting global sea lanes," said Gary Roughead, a veteran former admiral who retired last year as Chief of U.S. Naval Operations and now distinguished visiting fellow at Stanford University's Hoover Institution.
"But I do think there is going to be a very considerable policy debate - and probably a good one to have."
The challenge from an increasingly assertive China is becoming more obvious. Even with double-digit defense budget increases, Beijing's growing fleet remains well behind that of Washington, both in numbers of major ships and capability.
But China's announcement on Sunday of landings on its first operational carrier the Liaoning - a reconditioned Soviet era vessel purchased from Ukraine ostensibly for use as a casino - will unnerve some of its already jumpy neighbors.
With the exception of a small force in the Indian Ocean to counter piracy, China's entire naval focus remains on its immediate neighborhood - the South China Sea, particularly Taiwan, and disputed waters with Japan, Vietnam and others.
The United States, in contrast, finds itself stretched much thinner as it spreads its forces around the globe. If it is to follow through on its "Asia pivot" and match Beijing in its backyard, it may have to decide which other areas of the world to ignore.
The retirement this month of the USS Enterprise after half a century of service will bring Washington once again down to 10 carriers. With maintenance and training requirements, however, it can often only call on half that number at any given time.
Keeping one pair in the Gulf and another in Asia, experts say, could prove ultimately unsustainable.
That does not particularly worry naval officers who have long juggled limited resources around the globe. But it may force U.S. policymakers to moderate the expectations they have of both their own fleet and that of allies.
Europe's only "super carrier", the French "Charles de Gaulle", has also spent much of this year in refit after last year's Libya campaign. Italy and Spain have much smaller carriers, while one-time naval superpower Britain has none after scrapping its three vessels as part of major defense cuts.
Two larger British carriers, the "Queen Elizabeth" and "Prince of Wales", will enter service towards the end of the decade.
Having spent the last decade experimenting with landing aircraft on a ground-based mockup of a carrier flight deck, Beijing is clearly keen to make up for lost time. Several domestically built carriers are now under construction.
"The balance is clearly moving in the direction of emerging economies," says Christian le Mierre, naval analyst at the International Institute for Strategic Studies in London.
"There is a lot of focus on the Chinese carrier but people tend to forget India will also be operating three carriers within a decade."
Aircraft carriers alone do not define the strength of navies, he said. The U.S. Navy also has an unmatched number of amphibious warfare ships and other vessels from which you can deploy helicopters, vertical takeoff aircraft and drones as well as Marines and special forces.
With carriers in short supply, such ships have been become increasingly important. Several were sent into the eastern Mediterranean last week.
Washington can also use a variety of land bases, such as a base in Djibouti from which it is widely believed to have launched special force operations into Somalia and perhaps elsewhere. Submarines can hit targets well inland with missiles.
Even there, however, China is believed to be starting to close the gap. Earlier this month, it announced it would be sending nuclear ballistic missile-carrying submarines to sea for the first time.
For now, the U.S. Navy's approach to the world remains broadly unchanged. Where there is trouble, they will send additional forces moved from areas they hope will remain calmer.
Having kept at least one aircraft carrier in the Mediterranean throughout most of the Cold War and Balkan conflicts of the 1990s, in recent years the United States had quietly pulled back.
That, however, is now changing as the increasingly chaotic aftermath of the "Arab Spring" has brought instability to Egypt, conflict to Libya and Syria, Al Qaeda militancy to Mali, and further complicated this month's conflict in Gaza.
Earlier this year, Washington announced it would move four destroyers to the Spanish port of Rota and analysts expect a heightened presence elsewhere in the region - although a permanent carrier presence is seen as simply unachievable.
The military planning of all other major powers, experts say, almost invariably assumes the United States will continue that global approach. Even potential foes such as China are effectively dependent on U.S. naval power keeping global trade routes open.
In reality, however, other states may have to step up more quickly than they ever expected.
The global response to Somali piracy - in which the European Union, NATO, China, India Japan and others sent separate forces informally organized through meetings and an Internet chat room - might be a clue to the future. Shipowners welcomed the naval deployments, but have increasingly taken matters into their own hands by hiring armed guards.
"The future is going to be a lot more ad hoc coalitions," says Gvosdev at the U.S. Naval War College. "Others may have to take up the slack much more than they had expected."
(Reporting By Peter Apps; Editing by Ralph Boulton)