The intense diplomacy of the past few weeks has lifted the curtain on who does what for the President in national security matters. Harold Brown emerges as a Secretary of Defense who gives Carter live options by holding the Joint Chiefs and their assorted allies at bay.

Zbigniew Brzezinski, the President's special assistant, has saddled himself with the task of justifying the ways of Carter to mankind. Which leaves the way open for Secretary of State Cyrus Vance to come forward as the President's chief adviser.

Brown's case is probably unique in Pentagon annals. As a former director of research and Air Force Secretary, he comes to the top defense job with enormous technical expertise and great savvy in both congressional and bureaucratic politics.

He has missed no good opportunity to come front and center as the champion of the uniformed military and their tribunes in the Congress. Thus he went to the mat with the European allies the other day on the neutron bomb, even though it is a weapon of little importance.

Similarly, he found a means to postpone until 1980 President Carter's decision to pull troops from South Korea. When the director of the Central Intelligence Agency, Adm. Stansfield Turner, cast a covetous eye on Pentagon operations, Brown named another four-star admiral, Daniel Murphy, as his top intelligence man.

At the same time, Brown uses his expertise to hold the balance against military pressures embarrassing to the administration. He cleared the way for the President's decision against the B-1 bomber by developing a big case for the air-launched cruise missile. He has consistently helped the President on arms control by deflating scare talk of new Soviet developments in civil defense and nuclear weapons.

In consequence, Carter and his advisers have had the backing of the Joint Chiefs on such touchy matters as Panama, the B-1, Korea and arms control. Up to now, the administration has had no serious headache in the Pentagon.

Brzezinski plays almost as distinctive a role as special assistant on national security affairs. As he told Dom Bonafede of the National Journal. "The President wishes me and my staff to play an innovative role, that is to say, to try to look beyond the problems of the immediate and help him define a larger and more distant sense of direction."

In practice that has meant marrying to established foreign policy special themes that he developed for Carter during the presidential campaign. As it happens, many of the themes so important to the campaign have not fared well in the real world.

Thus "human rights" has had to be subordinated to better relations with the Soviet Union. Apart from Africa, which has beome the special province of U.N. Ambassador Andrew Young, good "North-South relations" have chiefly borne fruit in the failure of the Paris conference between developed and underdeveloped countries. The "trilateral" emphasis on relations between the United States Europe and Japan has resulted in a failure to promote economic growth, and a quarrel over the uses of nuclear energy for generating power that is only now beginning to be resolved.

Moreover, drum-beating by Brzezinski and his staff for some specific initiatives has misfired. White House emphasis on the special importance of the Palestinians to a Mideast settlement has caused much head-scratching among both Israelis and non-Palestinian Arabs. A White House claim that "progress" was being made in relations with Peking drew from the Chinese leader Teng Hsiao-ping a sharp rebuke accompanied by the charge that Sino-American relations were actually retrogressing.

Vance did not immediately seize the chance offered by these conditions. He was traveling constantly at first and was snowed under by paperworks. But at all times he stressed the need to move slowly and steadily, and he tended to subordinate such matters as human rights, trilateralism and the North-South connection.

Recently he has been concentrating with success on the major business: an arms-control agreement with Russia and a settlement in the Mideast. His recommendation to call off a meeting with Soviet Minister Andrei Gromyko projected for Vienna in early September yielded concessions from Russia when Gromyko visited Washington on Sept. 22. Those concessions point to the prospect of an arms-control deal before the end of this year.

The Middle East negotiations have been tougher - largely because of the public prominence given to the role of the Palestinians. But if the Palestinians pull out of the negotiations, which is not unlikely. Vance could luck into a successful Geneva conference on the Mideast.

For my own part I hope so. The administration will perform better - and look and sound even better - if the Secretary of State can assert himself as the dominant figure among the President's foreign-policy advisers.