Secretary of State-designate Alexander M. Haig Jr., facing the Senate Foreign Relations Committee in the opening hearings on his nomination, declared yesterday that even in the nuclear age "there are things that we Americans must be willing to fight for."

"There are more important things than being at peace," said the retired four-star general, quoting Patrick Henry on "liberty or death" and emphasizing the importance of national will and national strength in an "unusually dangerous" world. He repeatedly emphasized the importance of the U.S. military deterrent, which rests on potential U.S. willingness to exert its power, as the best means of preserving peace.

Haig, a former White House chief of staff and North Atlantic Treaty Organization commander, set out a powerful role for himself as the unchallenged "general manager of American diplomacy" in the administration of Ronald Reagan. In response to questions he announced it is Reagan's "firm intention" for him to be in charge and for the White House national security adviser, Richard V. Allen, to fill "a staff role for the president."

The U.S. foreign policy that Haig will administer, to judge by the six hours of testimony in a kleig-lit hearing room, is intended to be more muscular and more cohesive than that of the Carter administration. While touching on his aims, intentions and attitudes in a globe-girdling series of statements, however, Haig spoke only in general terms, declining to commit himself to substantive actions on grounds this is "premature."

Despite some advance expectations, the committee's inquiry in this first of several days of hearings concentrated far more on Haig's views on a broad range of foreign policy questions than it did on his controversial role in some aspects of the Nixon administration, including wiretapping, Watergate-related maneuvering and the choreography of Richard M. Nixon's pardon.

Sitting alone at a long table without notes or files other than his lengthy prepared opening statement, Haig answered with crisp confidence and an occasional raising or lowering of his voice in almost theatrical fashion. A few members of his family and aides, plus his attorney, former Carter Cabinet officer Joseph A. Califano Jr., sat just behind him.

Led by Sen. Charles Percy (R-Ill.), who has assumed the committee chairmanship with Republican control of the Senate, several lawmakers announced in advance that they expect to vote for Haig's confirmation. Even skeptical Democrats -- who have reversed their places with the Republicans to take over the minority seats for the first time in 25 years -- expressed their expectation that Haig will be confirmed.

Given the great importance of Haig's probable role, and the large number of volatile and dangerous foreign policy problems, the committee seemed to be going to unusual lengths to build a record of seeking answers.

Displaying his long experience in the corridors of powers, the general-turned-diplomat proved to be a canny witness, asserting himself with broad brush strokes and occasionally doing battle in philosophical terms while avoiding the pitfall of commitment.

He said virtually nothing about his view of the Iran hostage problem, which had dominated U.S. diplomacy for more than a year, and did not even list it among the urgent problems facing the nation.

He broadly hinted that he prefers to continue the U.S. grain embargo against the Soviet Union, saying that it now must be dealt with as a fact, but stepped away from a definitive statement, noting that Reagan has "his own track record" (against the embargo).

On another issue complicated by Reagan's past stands, Haig pledged to support the provisions of the Panama Canal treaty, which the president-elect strongly opposed during the debate over its ratification. But he also declared with irritation in his voice that "I haven't made a value judgment at all" about the merits of the treaty and would not say what he would do about attempts to undermine it by withholding funds.

Asked to list the most urgent and dangerous problems facing U.S. policy, Haig named the Soviet threat to Poland, "with the most grave potential consequences for world peace;" the Soviet actions in Afghanistan; problems involving developing nations; the growth of terrorism worldwide; international economic problems, and the conflicts in Latin America and the Caribbean.

In his prepared testimony he spoke of the concept of a common "third world" of developing nations as a dangerous myth, though he himself fell into using the term under later questioning. Haig criticized The Carter administration for failing to tailor its policies to individual circumstances of these nations, and for a "propensity to apply to these emerging states western standards."

At several points he criticized the application of human rights policy by the Carter administration. "I don't like to think it's naivete or stupidity that caused those mistakes, but perhaps an excess of zeal on the one hand combined with what is probably an inadequate mechanism for the application of it," he said. At the same time, he said human rights should continue to be high on the national agenda, calling this a "vitally important" concept for Americans.

The main elements of his testimony involved a more cohesive and vigorous U.S. policy in response to what he called "the central strategic phenomenon of the post-World War II era: the transformation of the Soviet military power from a continental and largely defensive land army to a global offensive army, navy and air force fully capable of supporting an imperial foreign policy."

Haig declared that the growth of Soviet strength, together with an "episodic" response from the West, had produced "perhaps the most complete reversal of global power relationships ever seen in a period of relative peace."

To counter this, he called for the emergence of three qualities in U.S. foreign policy:

Consistency, to replace a tendency to "react to events as they occur -- serially, unselectively, and increasingly in the final analysis, unilaterally." Haig said that the United States must realize that "the specific issues facing us today are merely surface manifestations of more fundamental problems."

Reliability, which is required to assure friends and to induce adversaries to act with prudence. "American power and prestige should not be lightly committed, but once made, a committment must be honored," he said.

Balance, in both the approach to individual issues and the orchestration of general policy. Haig said that balance must be struck in the selection and priority of issues, and the linkage between one issue and another.

Expressing his view on the application of this "linkage," Haig declared that "we must make clear to the Soviet leadership that they cannot expect benefits in a number of functional areas -- whether it be arms control, trade, credit transfer, technology transfer -- while they are indulging in activity worldwide which is endangering world peace." He added, "And I know that the leaders of the Soviet Union will understand that message."

He was not specific about what means he would use to combat Soviet power, but answered several questions that bear on the question.

Regarding the War Powers Act, which gives Congress a role in the dispatch of American military forces to combat zones, Haig pledged to comply fully "with the spirit and the letter," but noted that there are "ambiguities" and differences of opinion on how it applies in practice.

Asked about covert operations by U.S. intelligence or paramilitary units, Haig at first declined to reply in public session but, when pressed, pledged to comply with recent legislation requiring reports of such activities to the Intelligence committees of Congress. Haig said the new law by implication endorses cover operations "under the most unusual circumstances" and that "we [will] share the burden together" if such actions are ordered.

Haig said he is, in general, "comfortable" with Carter administration efforts to build up U.S. military power near the Persian Gulf, but said it is also important to scrutinize what commitments and consequences could flow. He declined to say whether he favors a U.S. military presence in Somalia, one of the facilities that the Carter administration has sought.

Haig expressed support for President Carter's "tremendous achievement" of an Egyptian-Israeli pact at Camp David, and said a "Jordanian option" to bring Jordan into the process under a new set of arrangements should be undertaken with the closest consultation of the current parties.

The former NATO commander said he was "disturbed" by the response of the United States and most of its allies to the Soviet invasion of Afghanistan a little over a year ago. He said the Soviets might not have taken the action if the West had been more vigorous in its opposition to the coup two years earlier that brought a Marxist regime to power.

In response to several questions about his views on nuclear weapons, Haig said a nuclear war would be "a disaster" but said that nuclear weapons play a major part in the U.S. deterrent, and thus could not be foresworn.

Asked if he at any time favored using nuclear weapons in the hostage crisis with Iran, Haig snapped back: "The very act of definitizing an answer to this question undercuts the fundamental deterrent on which our peace and security rests. I am not going to indulge in it."