Unlike some of his predecessors who were ambivalent about the United Nations or were bombastic and intimidating, Pickering's low-key approach is well-suited to the growing collegiality of the world body, his admirers say.

"He brings almost a boyish enthusiasm to the job," said Edward C. Luck, president of the independent United Nations Association. "He is excited about being there. He is excited about the potential of the U.N. And he realizes he's there at a very historic time."

Pickering, 59, does not dispute those characterizations.

"I've always believed that if you like your job and enjoy it, you ought to do it with some verve," he said last week after the U.N. Security Council voted 13-0 to authorize the use of force to back up its economic sanctions against Iraq.

The architect of the resolutions that form the legal foundation for the U.S. military buildup in the Persian Gulf is a balding, bespectacled man who is said to be fond of driving fast cars and participating in archaeological digs.

U.N. observers say that Pickering's disarming manner and sharp intellect - he is fluent in French, Spanish, Arabic, Hebrew and Swahili - were especially valuable in lining up support in the Security Council, where the nonaligned countries historically have resented the sway of the superpowers.

"In dealing with small countries, he seems to have just as much time and seems quite clued in to the issues of the region as he is with the larger countries," said Chan Heng Chee, Singapore's ambassador.

Thomas Reeve Pickering has made few enemies in his 30-year diplomatic career, although he was the target of assassination threats by right-wing death squads when he was ambassador to El Salvador from 1983 to 1985. And Sen. Jesse Helms (R., N.C.) sought to have him dismissed for opposing El Salvador's right wing by assisting the 1984 election campaign of Jose Napoleon Duarte, the Christian Democrat.

Pickering's understanding of the Third World grew out of his postings to the developing world. In addition to El Salvador, he has served as ambassador to Jordan, Nigeria and Israel. When he finished his post in Jordan in 1978, King Hussein called him "the best American ambassador I've ever dealt with."

He also served as assistant secretary of state for Oceans and International Environmental and Scientific Affairs, and was executive secretary and special assistant to Secretary of State William P. Rogers and then Henry A. Kissinger.

When Bush selected him in 1988, Pickering was one of four people who held the super-rank of "career ambassador," the highest rank in the foreign service. Bush, who himself served as ambassador to the United Nations from 1971 to 1973, said that the appointment was intended to symbolize the importance of the diplomatic corps.

Pickering is only the third career Foreign Service officer to be named chief of the U.S. mission at the United Nations. The other two were Charles W. Yost, an appointee of President Richard M. Nixon, and Donald F. McHenry, an appointee of President Jimmy Carter.

Pickering's service contrasts with that of his predecessor, Vernon A. Walters, who frequently went off on diplomatic trouble-shooting missions for President Ronald Reagan. And Pickering keeps a lower profile than Jeane J. Kirkpatrick, who was appointed during Reagan's first term, and Andrew Young, who was appointed by Carter. Some say that Kirkpatrick and Young distinguished themselves more for their grandstanding than for their U.N. service.

"In appointing a career diplomat to this job, rather than someone who was going to use it as a forum, say, for their political ambitions or a forum for their ideological convictions, the U.S. indicated it was going to take the U.N. more seriously," said Peter S. Wilenski, Australia's U.N. representative.

Pickering said he adapted easily to the cloakroom style of diplomacy that characterizes the United Nations - particularly the Security Council sessions in which "informal consultations" take on the appearance of a cocktail party.

"It's a little like the Congress, so you have to count noses," he said. "You have to make sure the people who are going to participate in the vote feel they're part of the process, and you do your best to bring them along."

Those skills as a negotiator proved valuable recently as Pickering worked late into the night over several days to build a consensus on the resolution that permits the use of force against Iraq.

Ever the diplomat, Pickering credits everyone but himself for the success of the U.S. initiatives - especially Secretary of State James A. Baker 3d, who persuaded Soviet Foreign Minister Eduard A. Shevardnadze to support the resolution.

"It was a question of trying to organize all those different facets of the issue and being patient and being ready to listen to the ideas of other states that helped us solidify the vote," he said.

In the end, the unanimity of the Security Council surprised most observers. Only Cuba and Yemen abstained. Even China, which had been expected to abstain, voted for the resolution.

After the vote, Pickering spoke firmly and precisely from his seat at the Security Council's horseshoe desk as though to reassure the other nations of the action that they had just taken.

"History will judge us by our resolve in the face of Iraq's threats to international peace and security," he said.