NOT since 1949, when the United States Navy made aircraft carriers and submarines the mainstay of the fleet, has the Senior Service faced as daunting a decision as it currently does.In the next two years, the Navy must set the mix of ships it will sail on the world's oceans well into the next century. By the Navy's own reckoning, the US fleet will drop from its 1980s high of 565 ships to 450 by 1995. Even if present shipbuilding plans proceed, experts say the Navy will shrink to between 410 and 425 ships by the end of the decade. A recent Congressional Budget Office (CBO) study puts the reduction as low as possibly 310 by the year 2010. "In the short term, the Navy will have to do more with less," says Ronald O'Rourke, a naval affairs analyst with the Congressional Research Service. The failed coup in the Soviet Union as well as President Bush's most recent statement on nuclear arms reductions "established a new defense spending climate," he says. Fleet levels in the Navy's 1995 plan are "unsustainable, mere way points on the way down," according to Mr. O'Rourke. Fleet structure is necessarily a long-term concern, says Anthony Cordesman, national security adviser to Sen. John McCain (R) of Arizona. The Navy mortgaged about 25 percent of its shipbuilding budget through the end of this decade when it decided to buy a new class of attack submarines, called Sea Wolf, O'Rourke says. Each sub costs $2 billion-plus. But by the end of the decade there will be a much greater need for amphibious assault ships to support expeditionary forces than attack subs, says Mr. Cordesman. The current inventory of assault ships is nearing obsolescence. In addition, the Navy has to make a clear decision on whether to buy a new $4.5 billion Nimitz-class nuclear carrier by next year, he says. "Before you build a fleet you must know the mission," says James Tritten, an associate professor at the Naval Post Graduate School in Monterey, Calif. Today's Navy, he says, has 2-1/2 fundamental missions. It must still provide deterrence by projecting a global presence, and it must be prepared to fight in a wide mix of coastal environments, as it recently did in the Gulf war. It also must keep an eye on the Soviet fleet. "Even though the Navy won't have to be ready, as in the '80s, to fight fleet-on-fleet battles with the Soviets on a day's notice, the Navy hasn't been quick enough to sort this out" in its fleet planning, says Jay Kosminsky, deputy director for defense policy at the Heritage Foundation, a Washington think-tank. The admirals are trying to protect expensive programs like the Sea Wolf, and "chances are they might not even get this," he says. The fact that this year's Naval Academy plebes who go into aviation will end their aviation careers before a new, long-range attack aircraft ever lands on the flight deck of a carrier shows how long the lead times are for ship and aircraft procurement, says Cordesman. According to the CBO study, the fleet will go from 15 to 8 carriers and from 80 to 40 or 50 nuclear attack submarines by the year 2010. The 18 Trident ballistic missile submarines are not likely to be touched, since they form the third, and most secure, leg of the nation's strategic triad. Whatever fleet configuration the Navy comes up with must fit the military doctrine of "jointness," (shared, mutual operations among the Army, Air Force, Navy, and Marines) proven successful in the Gulf War. The doctrine runs counter to the Navy's long tradition of independent action at sea. None of the services necessarily like the concept of jointness, says Mr. Kosminsky, but the Army and Air Force have learned to "phrase budget requests in terms of jointness." The Navy is still learning, he adds. Why has the Air Force been flying a Stealth fighter for a decade, and the "Navy still doesn't have one on the drawing boards?" he asks. The answer to that question is easy, says John Lehman, who was secretary of the Navy in the Reagan administration. "The Bush administration is carrying out the most anti-Navy policy since Truman," he says. Lehman criticizes the short-sightedness of a defense policy that evenly distributes the pain of defense cuts - one-third Army, one-third Navy, one-third Air Force. What is the mission of America's military forces? he asks. If it is likely to be a maritime/expeditionary role, as he posits, then it makes little sense to hamper the effectiveness of the Navy. Lehman likens the prospect of a reduced carrier force to a return to the "policies of the Repulse and Prince of Wales," two British warships sunk within 24 hours of each other in World War II by land-based Japanese aircraft. "Without lots of carriers you do not have air superiority over an area. You either leave that area or see your ships sunk," he says, especially in a regional or coastal conflict.