After 15 months of preoccupation with Central America, the Middle East and Poland, the Reagan administation now is engaged in an effort to increase U.S. and allied military strength in Asia as well, and reassure allies there they have not been forgotten.

The public centerpiece of this has been the 10-day swing through Japan, South Korea and the Philippines by Defense Secretary Caspar W. Weinberger that ended yesterday.

Behind the scenes, according to informed sources, the administration meanwhile is carrying out "at the very highest levels" a review of a proposed new "flexible operations concept" for the U.S. armed forces, especially the Navy. This reportedly would reduce, wherever possible, outdated commitments to keep specific numbers of ships in specific places and instead leave them free to roam more widely, especially in the Pacific.

This was Weinberger's first trip to Asia as defense secretary. The message he carried to all three countries was that, despite the withdrawal from Vietnam, the United States would remain a Pacific power and was firmly committed to help defend the region.

But Weinberger also told the allies, and stressed again to reporters on the plane back home, that the United States cannot defend Asia alone.

South Korea, with heavily armed forces and a big defense budget, got a pat on the back from the secretary during his visit.

The Philippines, which houses two huge American air and naval bases that support the Pacific and Indian Ocean fleets, also came in for praise from Weinberger, who first visited there 40 years ago as an infantryman in World War II.

But the message to Japan was different. Although pleased with this year's big 7.8 percent increase in the Japanese defense budget, Weinberger stressed that much more would be needed for many years and that an effective common defense with the U.S. fleet of the northern Pacific required that Japan take on a far greater role in its own self defense and its own self interest. He came away however, with no promises.

In an interview in Honolulu Saturday, Adm. Robert L. J. Long, commander of all U.S. Pacific forces, said those forces are stretched thin and are some 30 percent below the level needed. "Today, we have a classic situation where we do not have enough naval forces in the Pacific to perform all of the tasks given to us simultaneously," he said.

In addition, only two of the Army's 16 divisions and three of the Air Force's 26 tactical fighter-bomber wings are based in the Pacific.

While it is not unusual for military commanders to argue for more forces for their region, Long maintains that things have changed dramatically in the Pacific since the pullout from Vietnam a decade ago. Now there is a commitment to defend the Persian Gulf without the shah of Iran, to sail the Indian Ocean, and to contend with a Soviet fleet that has a third of its forces in the Pacific and access to a string of new facilities from Vietnam to the Horn of Africa.

The U.S. Pacific fleet, some 277 ships strong in 1973, now is down to 217 vessels. Only Marine Corps strength in the region, two divisions and air wings, has held steady.

"So this theater is no longer a benign one for the United States," Long says, adding that Asia "has also become the largest trading partner" for America.

Under the controversial and expensive administration plan to rebuild the U.S. fleet to 600 ships worldwide and to emphasize a more flexible, global strategy, some additional strength will flow to the Pacific.

But no matter how much is added, Weinberger has stressed that the United States will not be able to go it alone in a crisis.

Officials say privately that the administration has been lucky this first year in Asia with no crisis arising. But few believe it can sail through the next three years so smoothly.

That is why Weinberger called for help as well as offered it on his trip.

Japan, however, remains a question mark for the entire U.S. strategy.

Weinberger kept up the pressure on Tokyo to strengthen its self-defense forces in a manner that is appropriate under Japan's constitution but still an increase over the effort being put forth today. Washington wants a prosperous Japan at least to be able to defend its own homeland, beef up its anti-submarine and air defense capabilities and protect the sea lanes out to 1,000 miles.

A look at the map shows that a Japanese fleet could help bottle up in the Sea of Japan a large part of the Russian fleet based in Vladivostok. This would make it harder for the Soviets to get to the open sea and challenge the U.S. Seventh Fleet.

Weinberger says the Japanese understand the threat they face from the Soviets, who have a diviison of troops in the disputed Kuril Islands just north of Japan. U.S. officials say Japan has come a long way in expanding its forces in the last 10 years and that a new generation of younger officers believes the country, as another democracy, should be able to defend itself. The U.S. task, they say, is to encourage this trend without breaking whatever public consensus for defense is emerging in Japan by applying too much pressure.

Some U.S. government specialists, however, are skeptical that this can work, and are pessimistic about Japan's ultimate value as an ally if the chips ever are down.

In this view, Japanese leaders today do not fear the Soviets much and are likely to do only as much for defense as it takes to keep American pressure at a level that they can handle.

These officials say, therefore, that every American administration must keep intense pressure on Japan. Even if they do, however, officials believe Japanese defense progress will be glacial at best and Japanese forces cannot be counted on in a major way for many years, if ever.

Japan's small armed forces today are effective and the country obviously has huge potential for military strength. Yet here, too, Washington must walk a thin line because, as Weinberger concedes, Koreans, Filipinos and other Asians--along with the Japanese themselves--do not want a heavily armed Japan.

Thus the key question of whether Asia can be defended without a vastly larger role for Japan also goes unanswered, raising questions about whether added American strenght by itself in the Pacific will make much difference.

In addition, there is private unhappiness in Washington, and a lot in South Korea, that Japan has not been more forthcoming with economic aid for South Korea.

Despite an economy that has boomed in recent years, South Korea remains a tense country. It worries about its internal security as well as the threat from an even more heavily armed and menacing government in the communist half of the divided nation. The North Koreans have a 700,000-man army, including a special 100,000-man force believed to be the largest commando and infiltration force in the world.

South Korea is well armed and spends 6 percent of its gross national product on defense. But what worries U.S. planners is what would happen if a crisis elsewhere meant the United States could not reinforce South Korea if North Korea invaded again.

Thus, while Weinberger and South Korean President Chun Doo Hwan paid homage to the need for negotiations with the North Koreans, the overriding emphasis during the visit was on sabre-rattling to warn the neighbors to the north not to attack. The United States stressed the nuclear umbrella it holds over Seoul and announced new procedures for turning over $2 billion of U.S. war reserves in an emergency.

Although most American analysts do not believe the threat of a new war in Korea has increased, there are 40,000 American troops there, many of them in the path of any possible attack. In other words, if war does come, the United States could find itself involved quickly.

Weinberger's trip to Asia came soon after what turned out to have been a controversial trip to the Middle East in which the defense chief discussed arms sales to Jordan and some of his aides embellished on those remarks to reporters. Perhaps to avoid such things, the press on this trip was provided with briefing books and officials other than Weinberger did very little talking.

This trip, therefore, was undertaken with some trepidation and considerable care.

By all accounts however the trip seemed successful. Weinberger handled himself and his hosts with considerable aplomb. Although the defense secretary has a talent for delivering good speeches badly, his messages were well received, produced serious responses and seemed to raise the general level of public consciousness in Asia, at least for a while, on security matters.