A leading civilian defense expert says that unless the United States broadens its strategy and pays more attention to the reserves and the National Guard, even the big military budgets proposed by the Reagan administration may not prepare the nation for the kind of battles it may have to fight.

William T. Kaufmann, a professor of political science at the Massachusetts Institute of Technology and the drafter of the Pentagon's annual defense posture report to Congress during the Ford and Carter administrations, said that while the final Carter and the new Reagan defense budgets are a step toward meeting standard dangers, "they do relatively little to ensure against a more testing yet not implausible set of dangers."

Kaufmann argued that the real challenges to the U.S. military in the years ahead are more likely to involve conventional, rather than nuclear forces.

The size of those forces and where they are stationed are still based on a strategy of having to deal with one major and one lesser confrontation simultaneously and being able to move troops, ships and planes around, Kaufmann said. This strategy, he said, is "open to question."

In a 50-page section of the new Brookings Institution analysis of the 1982 budget and national priorities, and during a meeting with reporters, Kaufmann argued that the most serious non-nuclear contingency facing the United States is the possibility of almost simultaneous military challenges in the Persian Gulf, central Europe and Korea.

Under those circumstances, Kaufmann said, the United States would find itself short by at least four divisions of ground troops and six tactical fighter wings. If the Caribbean erupted as well, he said, the deficit would be worse.

He said he believes, however, that those forces could be quickly supplied by three Army reserve divisions and one Marine reserve division, plus Air Force reserve squadrons, if the Army would bring those reserves to a high state of readiness and the Pentagon would buy better equipment for the highly trained air reserves.

As matters stand now, Kaufmann said, it would take the Pentagon more than 90 days to get any major organized unit out of the Army National Guard or reserve divisions, other than those reserve battalions already counted as part of active-duty divisions.

He said it is politically easier and less costly to get the extra forces from the reserves rather than from the draft. The United States is paying about $11 billion a year for the reserves and not getting much for it at the moment, he said.

In laying out what he sees as U.S. options, Kaufman argued that vast sums requested by the new administration for long-range spending, beyond the initial budgets now before Congress, are in effect "wedges," money in reserve for which no specific use has been identified.

"For those who must translate these various signals into programs," Kaufmann wrote, "a certain caution about defense needs is in order."