When Thomas C. Green decided to represent retired Air Force major general Richard Secord in the Iran-contra hearings, he relinquished another client -- Lt. Col. Oliver North.

North soon had a new lawyer -- Brendan V. Sullivan, who had worked with Green on several cases. The man of action had placed himself in the hands of a litigator so fierce that his tactics have been described by colleagues as the legal equivalent of "nuclear war." "He asks no quarter and gives no quarter," says Plato Cacheris, the lawyer representing Fawn Hall.

Sullivan, who will be at North's side this morning when his client appears before the House and Senate select committees investigating the Iran-contra scandal, is a senior partner in Williams & Connolly, perhaps the leading criminal law firm in the country. His friends and associates say that the "tenacious" and "indefatigable" Sullivan works "inhuman hours," consumed with "an unyielding commitment to winning." "I can usually reach Brendan at 7:30 a.m. in his office," says Arthur Liman, the chief counsel to the Senate select committee.

"He has a soft, warm personal manner," says Arnold Wiener, a Baltimore lawyer. "But one millimeter below the surface, he's steel."

Following the Williams & Connolly dictum, Sullivan immerses himself in the facts of a case, becoming more conscious of its details than even the client. "He extracts everything that can be extracted from a case," says the firm's founder Edward Bennett Williams. And on the squash court, Williams adds, he is "just as tenacious."

"In a world where there are few points of reference, Brendan is in fact that," says restaurateur Richard McCooey, a friend for 25 years. "If you had a problem and you needed to have protection, you'd go to Brendan Sullivan and feel incredibly secure that he is defending your rights. It's a wonderful sense of confidence and security when you are in trouble and everything looks bleak. Brendan would say, 'Calm down, we're going to take this step by step.' "

While all lawyers seek to frame the issues in a case favorably for their client, Williams & Connolly is renowned for its brilliant packaging, for casting a drama in which the denouement is the client's happy ending. In the matter of John Hinckley Jr., the would-be assassin of President Reagan, for example, Williams & Connolly lawyer Vince Fuller successfully argued the difficult insanity defense by depicting a young man whose mental disturbance made him appear incapable of rationally carrying out the assassination.

In the matter of Oliver North, Sullivan's scene setting is complicated because the client faces a double drama: the klieg lights of the congressional hearing and the likelihood of indictment.

"Brendan told us very bluntly that he would have preferred that North not be called at all," says Liman. "Brendan did not seek us out until the day we called him up."

Sullivan had to maneuver adroitly with the committee. If North had refused to testify, he faced being cited for contempt of Congress; at the same time, the main event for North and Sullivan has always been the indictment sought by the special prosecutor.

In this two-front war, every action Sullivan's client takes in one forum has been with an eye on the other. For example, as a preemptive strike, Sullivan filed a suit challenging the legality of the office of special prosecutor. "Brendan is tenaciously innovative," says Robert Barnett, a Williams & Connolly colleague. "He's never afraid of a new theory or taking on the government. He will go with an issue all the way."

From the committee, Sullivan negotiated an exchange of North's testimony for limited immunity. The prehearing testimony, moreover, was kept to questions about what President Reagan knew and when he knew it. "Brendan argues very, very logically," says Liman. "He doesn't come in making an emotional pitch. People are always coming in crying. You know what Brendan's looking for."

As a measure of Sullivan's control, the loquacious North has been silent until today. Sullivan himself has refused to return all requests for interviews, as he has for this article.

When the hearings are over, only the first act of North's drama will have concluded. The "scorched earth" campaign that the attorney has learned to practice with perfection will continue.

Brendan Sullivan, 45, grew up in a small town near Providence, R.I. His father, a chemical company vice president, was also the local police commissioner. Brendan attended Georgetown University and Georgetown Law School. Though he has never been what is called "a drinker," he led the movement to change the regulation forbidding consumption of beer on campus -- perhaps his first successful case.

With his friends he celebrated victories over exams at the 1789 restaurant, where a brass plaque was mounted, inscribed with their names and a Latin saying: "On this day we have really lived."

His loyalty to Georgetown was abiding, and he became one of its counsels. (Edward Bennett Williams is also an alumnus.) Also abiding was Sullivan's devotion to the restaurant and its proprietor, Richard McCooey, who became a friend.

Since Sullivan graduated from law school, his life has been a series of cases.

The one that brought him to Williams' attention occurred in 1969. At the Presidio Army stockade in San Francisco, a 19-year-old private was shot to death running away from a work detail. Three days later, 24 soldiers staged a sit-down strike, singing "We Shall Overcome" and "America the Beautiful" while an officer attempted to read Article 94 of the Uniform Code of Military Justice dealing with mutiny. They were all arrested and charged with the crime.

Capt. Brendan Sullivan volunteered to defend them. He took on the case, according to a friend, because it was difficult. But the Presidio provost marshal charged he was trying to turn the trials into "some sort of antiwar circus." During the proceedings, about six months before Sullivan's Army service was completed, he was ordered to Vietnam. Senators and Walter Cronkite were upset, and an investigation was threatened. The secretary of the Army himself rescinded the order. And Williams' interest in the young lawyer was piqued. Sullivan was promptly hired.

With the firm, Sullivan became involved in several celebrated cases. In 1973, he defended Alan Green, the Baltimore engineer who had paid $87,500 in bribes to a politician who had risen from Baltimore County executive to Maryland governor to vice president -- Spiro Agnew.

Agnew pleaded nolo contendere and served no time. Green, who cooperated with the prosecutor, received one year. Two other cooperating witnesses escaped prosecution. The deal of testimony for a plea bargain was made, according to a source, because Sullivan was apparently led to believe that the prosecutor would recommend probation. Unfortunately for Green, there was no such recommendation.

In 1978 Sullivan defended a Catholic priest, Guido John Carcich, a direct-mail fund-raising wizard, charged with embezzling $1.4 million and secreting $15 million in bank accounts -- all money raised for the poor in the name of the Pallottine Fathers, a small priestly order.

In Sullivan's brief, the wheeler-dealer was described as a man whose "list of good works . . . is so numerous as to defy recounting." The brief also conceded that Carcich was "guilty of the crime charged." Behind the scenes, a bargain had been worked out. Carcich pleaded guilty to one charge; the other 61 were dropped. He served a year at home and is now a parish priest in North Carolina. The Maryland attorney general, Francis Burch, who negotiated with Sullivan, dropped his campaign for governor because of the deal's unpopularity.

In 1984, the Justice Department issued what seemed to be an airtight indictment of massive tax fraud against the Rockville-based Omni Corp. The airplane trading firm was accused of avoiding millions in taxes by funneling its income to an offshore subsidiary.

Sullivan suspected that the IRS had breached Omni's attorney-client relationship, but didn't know exactly how. He filed a motion to have the case dismissed, without having the proof in hand. The 28 days of hearings on the motion were used to get it.

When the government handed over IRS memos showing remarkable solicitude for Omni's attorney-client relationship, Sullivan's suspicions were heightened. A retired FBI agent hired as a private investigator and a Williams & Connolly colleague, Barry Simon, discovered that the IRS documents were typed on government bond paper that revealed the watermark of an eagle when held up to the light. Below the eagle was the year the paper was made -- "1984." But the IRS claimed that the memos were written in 1983.

In 1986, U.S. District Court Judge Walter E. Black Jr. said the government's conduct was "patently egregious and cannot be tolerated or condoned. Its manner of proceeding shocks the court's conscience." Case dismissed.

The big cases were his making, but the small ones did not receive less attention. For example, when a laundromat vendor sued Georgetown University for $10,000, the case was handled by a junior lawyer in the firm, Pierce O'Donnell, now a Los Angeles attorney. Sullivan asked if they could chat. "I thought I was going to have a routine 10-minute conference," says O'Donnell. "I got grilled by him for two hours: 'Have you thought about this? Have you thought about that?' The case could have been about 10 million dollars." The case, O'Donnell recalls, was won.

No gambit evaded Sullivan's imagination. When taking depositions in his law office from a hostile witness who smoked he would post beforehand an American Cancer Society warning about the mortal effects of smoking.

At the firm, Sullivan was known for his relentless labor, his unmatchable billable hours. He is often mentioned as the heir apparent to Williams. His wife Lila, a trained but not currently practicing attorney, says a friend, is supportive of his saturation in work.

Still, he enjoys his three children. The Sullivans have bought a house in Annapolis, where they keep a small yacht, fittingly named the Mistrial. And one year, he served as chairman of the Friendly Sons of St. Patrick, a mirthful group devoted to celebrating St. Patrick's Day at an annual dinner.

But Sullivan has never been driven by obvious motives. The unknown young lawyer who defended the Presidio soldiers may still be seen in the established Washington lawyer who now defends North -- a "pure attorney," in the words of Plato Cacheris.

"He's doing it for some mysterious thing," says a friend. "I'm sure he likes the money; it's a way of keeping score. But that's not it. It is not for ego. When your ego is it, that's when you make mistakes. To be the best, that's everything. He makes cerebral choices. The law gives him pleasure. If there's anything negative, it's that he gives too much to his clients, some of whom are worthy and some of whom are not."

For Oliver North, the beginning of the long march through the legal system is the end of ideology -- and the end of a picaresque career that has led him to Iranian arms brokers and Swiss bankers, conservatives and contras. Now what North says and does will be determined, as he noted at the congressional hearing where he invoked the Fifth Amendment, "on the advice of my attorney."