"Present at the Creation . . . ," read yesterday's half-page newspaper ad. Perhaps only in Washington would such a grandiloquent claim introduce not a new hotel or line of haute couture, but a journal of foreign policy.

"We're reasonably ambitious and confident," said Owen Harries, editor of The National Interest, a self-proclaimed neoconservative quarterly that will debut in about three weeks.

The National Interest could just as well have been called "Hail, hail, the gang's all here," though the chosen title will doubtless be taken more seriously. The masthead is a collection of the names that made "neoconservative" a household word.

Irving Kristol, co-editor of The Public Interest, is publisher. The advisory board boasts former ambassador to the U.N. Jeane Kirkpatrick, former chairman of the Council of Economic Advisers Martin Feldstein, writer Midge Decter, New Republic senior editor and columnist Charles Krauthammer, military critic Edward Luttwak and Henry Kissinger, who needs no introduction. The first issue features articles by Carter national security adviser Zbigniew Brzezinski and Assistant Secretary of Defense Richard Perle, among other academic and government figures.

"Irving Kristol and I both started to feel some time last year that the conservative movement in this country was becoming strong and various enough to need more intramural discussion," said Harries, a former ambassador to UNESCO from Australia and adviser to former Australian prime minister Malcolm Fraser. "We feel this period, rather like the late '40s, is a sort of seminal period; that we are, so to speak, between paradigms; that in the next few years, a new phase in the American foreign policy is going to emerge."

Hence "Present at the creation . . . ," Dean Acheson's modest phrase for his role in shaping American foreign policy after World War II, and the statement "You are invited to be part of a new magazine destined to exert a profound influence on American foreign policy."

"Up till now, there have been two journals of foreign policy," said publisher Kristol. "There's Foreign Affairs, which is an establishment, sort of center magazine, and there's Foreign Policy, which is slightly left-of-center. And now there's us -- the trinity.

"For years foreign policy was defined in liberal, internationalist terms. The hope was one could establish international organizations which would in effect substitute for a foreign policy. The U.N. is the best example of this. We are persuaded this era is over. There is not going to be a world community of nations, and countries will have to create their own foreign policies."

The newspaper ad offered prospective subscribers a pre'cis of what "we believe" at The National Interest -- including the assumption that defending and advancing the interest of the United States is "the purpose of American foreign policy," that "the efficacy of military power remains undiminished" and that "the Soviet Union continues to pose the greatest threat to America's interests."

"Americans are hard-headed and pragmatic in relation to domestic policy and politics," Kirkpatrick said yesterday, "but we have tended to see foreign affairs as a kind of domain for utopianism, for universalism, rather than the protection of national interests. American foreign policy has long tended to suffer from an inadequate conception of our national interest. I believe very deeply that the national interest should be the center of gravity of our foreign policy."

Krauthammer agreed, although he suspects he was asked to join the advisory board "to hold down the left wing" and felt he had "a moral obligation to do it." Rarely described as even residing near, let alone living in, the left wing of political thought, he nevertheless protested that "I'm not accepted in the neoconservative church. I've never been a member or even an official layman."

Just what church members believe America's "national interests" to be, Kristol would not venture to say. "The point of our magazine is to help define that," he said. "I don't think there is any set definition."

The Reagan administration, he said, "does many right things and many wrong things, but it almost never gives the right reasons for what it does and it never can, because the reasons haven't been articulated. The whole vocabulary, the very grammar as it were, of foreign policy has been underdeveloped over these past decades."

Expeditious grammarians who become charter subscribers can save $2.50 off the regular subscription rate of $18, proving that being in on a creation can be economically wise, as well as morally uplifting.