A staunch ally of Defense Secretary Caspar W. Weinberger who is known for his ability to succinctly sum up a complicated situation pondered long and hard when asked how the fiscal 1986 Pentagon budget would fare in Congress in the new year.

"The problem," this high official said, "is that Weinberger will be perceived as not giving as much at the office as the other department heads."

Because of that, the official predicted, the lawmakers will be eager to even things up by cutting the Pentagon budget more than usual.

However, the veteran of budget wars added, the politicians will find it much easier to talk about forcing Weinberger to give more at the office than doing it.

For one thing, about half of the Pentagon budget is politically untouchable. This is the personnel account, which includes pay for the troops. The multibillion-dollar weapons programs have too many jobs attached to them to appeal to most lawmakers as targets, the official continued, leaving the edges of the DOD budget to nibble at.

The edges include spare parts, ship overhauls and scores of little programs that few people have heard of but that add up to billions. Canceling the MX missile -- a possibility in the new year -- would be a big deal politically but relatively small potatoes in the budget. Only $1.5 billion in fiscal 1985 spending would be saved by canceling the MX, and less than $4 billion in fiscal 1986.

As 1984 came to a close, the Pentagon's chief spokesman was saying things that would have been heresy only four years ago but that now fit into the effort to persuade Congress and others that Weinberger indeed gave at the office -- even more than President Jimmy Carter would have given had he been reelected.

At his Dec. 13 briefing, spokesman Michael I. Burch read reporters a list of cuts made in the defense budget since President Reagan took office. The point was to show that the administration had ended up with a smaller budget than Carter had projected in his five-year plan for fiscal 1982 through 1986.

"So the point you're trying to make," asked one reporter, is that the Reagan defense buildup is "now less than the Carter administration proposed?"

"You can make that point," Burch replied.

Pentagon officials used to worry about what conservatives would say if Carter managed to look tougher on defense than Reagan -- at least as measured by Carter's final defense budget projections, which defense insiders considered unrealistically high. PEOPLE PROBLEMS . . .

While the leaders of the Army, Navy, Air Force and Marine Corps all cheer as they get billions to buy more weapons than at any time since World War II, they fret about people problems confronting them in 1985:

The Army is worried about not having enough tail behind its teeth -- meaning units that pass the ammunition, repair the tanks, deliver the fuel and repair the radios. Gen. John A. Wickham Jr., Army chief of staff, would increase the size of those units rather than form new divisions if he is given more people.

The Navy pays generous bonuses to aviators who stay in the service, but it still sees a pilot shortage ahead. Airline hiring, family separation and long carrier deployments are seen as reasons.

If the MX goes down and the Midgetman goes up, the Air Force will have to find nearly 100,000 people to operate, transport and guard the tiny missiles, which would be carted around military bases -- and perhaps civilian areas as well -- to make them hard to hit.

The Marines, now that they are adding light assault vehicles, sophisticated AV8B Harrier jump jets and new antiaircraft and antitank missiles to their once-spartan arsenal, need more than a few good men. The corps, now numbering about 196,000, will surpass 200,000 soon -- perhaps up to 204,000.