Defense Secretary Caspar W. Weinberger said yesterday that the United States should find ways to help guerrilla movements fight communist governments, but he again advocated caution before committing U.S. forces.

In a speech prepared for delivery at Fort McNair here last night, Weinberger kicked off a high-level "conference on low-intensity warfare," which Secretary of State George P. Shultz is scheduled to address today. Defense Department officials said the conference is intended to underline the Reagan administration's commitment to those it considers "freedom fighters" in Nicaragua, Afghanistan, Angola, Cambodia and elsewhere.

Citing those four nations and "others who look to us," Weinberger said that Americans "cannot ignore their aspirations without betraying our own."

"If it is proper and just that we should help those who wish to remain free," he said, "then we can hardly turn our backs on those who have lost their freedom and want it back."

But Weinberger, resuming a debate that he and Shultz have waged on the use of force, said that the military must be only one component of a strategy to defeat communism. That strategy must include "diplomacy, and economic leverage, and the proper management of our technological riches, and the proper, unashamed and unremitting willingness to make our case at the bar of public opinion," he said.

Shultz has argued in speeches that the United States should be more willing to back diplomacy with military muscle and to strike back at terrorists. Weinberger responded with a speech in November 1984 laying down six conditions for the use of force, including "reasonable assurance we will have the support of the American people and their representatives in Congress."

The defense secretary acknowledged yesterday that some have criticized that condition, but he said it is particularly important that U.S. soldiers enter low-intensity conflicts only with clear public support.

"What is important is that we never lose sight of the fact that the military is an instrument of the national will, and not a substitute for it," Weinberger concluded.

Weinberger's prescriptions appeared to fall short of those advocated by some conservative strategists. Later this month, for example, the Heritage Foundation will conduct a seminar entitled, "How to Roll Back the Soviet Empire Through Low-Intensity Warfare."

Weinberger acknowledged that even the term, "low-intensity conflicts," which has become increasingly fashionable among defense analysts, is fuzzy. Such Third World conflicts are sometimes labeled insurgencies, guerrilla wars or, in Soviet literature, wars of national liberation.

The defense secretary said "there is a place for power" in responding to low-intensity wars, particularly for special operations forces such as the Army's Green Berets. Even there, however, he stressed the value of special forces in training local fighters and performing "civic action," such as digging wells, building roads and offering medical help.

The Reagan administration has pushed fitfully for a more active role, both open and covert, in Third World conflicts, from which the United States pulled back after the Vietnam war. U.S. dollars are flowing to antigovernment forces in Nicaragua, Afghanistan and Cambodia, and Washington may soon resume aiding Angolan rebels as well.

The administration also has increased support for governments threatened by leftist insurgencies, as in El Salvador. Weinberger said last night that such aid should not go to a government that will use it only to "sustain itself in power, but he added, "We cannot permit our disdain for some imperfect regimes to bring forth far worse alternatives."