Like a field commander trying to hold a strategic hill against swarms of attackers, Secretary of Defense Caspar W. Weinberger is digging in for a battle this week that cuts to the bone of the financial and ideological underpinnings of the Reagan administration.

That battle will be with other key administration figures, especially White House budget officials and presidential aides. It will be over how much, if at all, to cut a five-year, $1.6 trillion military spending plan that is meant to symbolize a strengthened, tougher America but which also could wreck another administration goal of fiscal responsibility, lower taxes and balanced budgets.

In a speech to an American Legion convention in Honolulu Thursday, Weinberger made clear what he felt ideological purity in the Reagan administration is all about.

"The only sacrifice we must make today is to be prepared to be strong enough to deter aggression. That is the most important task this administration was elected to carry out," Weinberger, 64, told applauding legionnaires.

Later, in an interview aboard his plane flying back to San Francisco, Weinberger revealed his feelings in more detail.

"It is not a question of being immune from reductions," he said. "I have no quarrel at all with the theory that every department of government should be examined carefully and no department should be exempt.

"But the simple, sad fact," he went on, "is that because of the neglect of previous years, we do have a very large requirement in the whole defense field.

"We have to move on several fronts. There is the need to modernize and expand the strategic and conventional forces and to fill them out to enable us to play a major leadership role in the world. That is a heavy burden. But it is a burden that has to be shouldered."

Vietnam had caused "a long loss of momentum," he said. The Army, Air Force, Marine Corps and particularly the Navy all have to be strengthened to back up the more global military strategy the Reagan administration is now trying to shape, he said.

"All those things need to be done. They are long-term plans. They are defensive in nature and they are expensive."

If these military spending plans are not matched by reductions in domestic spending, it will be inflationary. If they are matched by a reduction in other spending, which the administration has been attempting to do, then Weinberger says "I don't think it will be inflationary."

"You can't wreck the economy doing this and you can't forget that it has to be related to our resources." But it becomes a problem for planners, industry and the military when a new administration starts to crank things up again, then must drop back.

Weinberger is a longtime, close friend of President Reagan, certainly among the most influential figures around the chief executive. Though always gentlemanly, he is also scrappy and quick to sense intrusions into his area of responsibility.

Thus, all last week Weinberger did not shy away from letting it be known publicly that the battle over the Pentagon's budget was not over until the last shot was fired and that it would be the president, not White House aide James A. Baker III or press spokesman Larry Speakes or budget director David A. Stockman, who would fire that shot.

Baker, then Speakes, talked about up to $30 billion in cutbacks on Weinberger's turf. Although Weinberger says he is not bitter, it seems clear he was surprised by Baker's pronouncements, viewed them as an invasion and resented such public lobbying and pressure tactics.

On his plane, Weinberger did not want to say anymore about it. "I think we've all said all we can on that," repeating that "no decisions have been made" on the defense budget.

"One of the problems with this whole business is we all," meaning both officials and the media, "are forced to keep talking about things before they are finished."

While Weinberger chatted with reporters traveling with him, preliminary drafts of what various levels of defense cutbacks would mean were being reviewed in Washington by White House officials and Weinberger's deputies.

Perhaps with that in mind, Weinberger seemed somewhat less certain about the dimensions of the military expansion that has been associated with the Reagan administration. Rather, Weinberger focused on the subject of making existing units completely battle-ready, a priority that many defense specialists in and out of government would applaud.

Administration strategy calls for the ability to combat the Soviets anyplace in the world rather than just where the Soviets might chose to attack. That strategy assumes larger forces than now exist to meet all the contingencies.

"Ultimately, I think we will have to have a larger capability of acting or reacting in various parts of the world," Weinberger said, "and what that means is that you've got to have units that are fully fleshed out.

" We can't have the kind of 'hollow Army' units anymore" that Army chief of staff Gen. Edward C. Meyer talked about last year "and have any kind of deterrent" which would keep a war from happening in the first place, Weinberger said.

However, "what we need to do first is to have a number of trained, fully equipped, ready-to-go divisions. We have some of that but not enough," Weinberger said.

Although Meyer has said the Army should have another two active divisions beyond the 16 already formed, Weinberger said, "I wouldn't commit myself to that yet because we want to be sure we don't just get more of a hollow army.

"My feeling has been all along that we could present to the public 16 or 17 divisions, but it wouldn't be really meaningful or indeed completely honest. What we need are fully-equipped units and if we are a couple brigades short in a standing peacetime Army that wouldn't bother me nearly as much as if we were three or four divisions short of being fitted out, equipped, trained and ready to go.

"There are two routes you can go," he continued. "One is to expand and hope you can fill them out. Or you can fill out what you have and the latter is what we are trying to do as quickly as available resources will allow."

Making what we have now completely combat ready "is a very high priority. That doesn't preclude some expansion, but if a choice actually had to be made, I think making completely ready what we have now is a vital part of it," Weinberger said.

"We don't want to expand unless we can do it in a way that is useful and effective," so it may be in the next five-year period that the Army ends up with one more division or perhaps just some brigades and battalions which are then filled out in a second five-year period.

Explaining why ultimately the United States may need more ground forces and a bigger Navy, Weinberger said, "We will have to have some kind of ability to resupply and reinforce" in many places.

"If, for example, the Soviets thrust through Iran and Iraq into the oil fields we would have to, if the host countries wanted us, go over there and prevent entry into the oil fields. At the same time, we have to be prepared for the fact that, in a very big global sense, that could be a feint" and that the main thrust might be coming through the central front in Europe.

Weinberger said the United States cannot cope with this by itself; the NATO allies and Japan must help. What also helps, he maintains, are the U.S. arms sales to Israel, Pakistan and Turkey.

The defense chief disputed critics who contend the United States needs simpler rather than more sophisticated weapons, saying the Soviets are following the latter route and U.S. units cannot be less well equipped.

He defended the neutron weapon as precisely one which can overcome Soviet advantages in tanks and a strategy that calls for simultaneous assaults at the front and rear of Soviet lines to limit Moscow's ability to reinforce or sustain an assault.

"You also can have a situation in which you try and engage an enemy on more than one front, in their areas of weakness. And those are some of the things, with the help of our allies should we get into any kind of conflict, we would be preparing to do," he said.

In the final year of the Carter administration, strategists also spoke of this "hit 'em where they are weak" strategy, but it has never been spelled out where the Soviets would be weak, except possibly Cuba.

Because of this global strategy, Weinberger laid heavy emphasis on the need for a strengthened Navy. But here, too, he was noncommittal about the need for 15 battle groups, each centered around present aircraft carriers and the three new carriers the Navy wants to build to reach that goal.

"What we have is a commitment to do things necessary to strengthen" the Navy, he said. "Whether we do another" carrier beyond the new one whose keel will be laid this fall, "I don't know," Weinberger said. "But I'm sure if we don't in this five-year period, we will in the next."