At Key Biscayne, according to a Secret Service source, Nixon once lost his temper during a conversation about Cambodia. "He just got pissed," the agent quoted eyewitnesses as saying. "They were half in the tank, sitting around the pool drinking. And Nixon got on the phone and said: 'Bomb the shit out of them!'"

"If the president had his way," Kissinger growled to aides more than once, "there would be a nuclear war each week!" This may not have been an idle jest. The CIA's top Vietnam specialist, George Carver, reportedly said that in 1969, when the North Koreans shot down a US spy plane, "Nixon became incensed and ordered a tactical nuclear strike... The Joint Chiefs were alerted and asked to recommend targets, but Kissinger got on the phone to them. They agreed not to do anything until Nixon sobered up in the morning."

The allegation of flirting with nuclear weaponry is not an isolated one. Nixon had been open to the use of tactical nuclear weapons in Vietnam as early as 1954 and as president-elect in 1968 had talked of striking "a blow that would both end the war and win it". A Kissinger aide who moved over to the White House, David Young, told a colleague "of the time he was on the phone [listening] when Nixon and Kissinger were talking. Nixon was drunk, and he said, 'Henry, we've got to nuke them.' "

The 1972 election, when Nixon won his second term as president, was, as predicted, a landslide. Yet it was from the beginning a peculiarly joyless triumph, the start of an administration doomed to disaster - as the Watergate affair unravelled and many of Nixon's closest aides were either prosecuted or resigned - and culminating, eventually, in Nixon's own resignation. The firing of Haldeman and Ehrlichman, press aide Ron Ziegler, thought, was Nixon's lowest emotional moment. "He looked out of the window and said, 'Ron, it's over.' And I knew he was referring to himself and the presidency."

Nixon actually spoke of resigning in the spring of 1973, 16 months before the final fall. "Maybe," he told Kissinger during one after-dinner phone session, "we'll even consider the possibility of, frankly, just throwing myself on the sword... and letting [Vice President] Agnew take it. What the hell." Kissinger told him not even to consider it.

Nixon brought up the subject of resignation twice more in the weeks that followed, with his family. Pat urged him to fight on, as did his daughters, arguing that the country needed him. Kissinger considered what he was seeing was nothing less than "the disintegration of a government that a few weeks earlier had appeared invulnerable. The president lived in the stunned lethargy of a man whose nightmares had come true... Like a figure in Greek tragedy, he was fulfilling his own nature and destroying himself."

In June, a year to the day after the Watergate arrests, when former White House counsel John Dean was about to testify before the Senate, Nixon received the Soviet leader, Leonid Brezhnev. He had insisted on going ahead with the summit, Kissinger believed, because "to concede that his ability to govern had been impaired would accelerate the assault on his presidency". Yet the fact was, in Kissinger's view, that Watergate had "deprived him of the attention span he needed to give intellectual impetus to SALT" - the arms control talks.

"By the end of the visit," Kissinger gathered from Soviet Ambassador Anatoly Dobrynin, "the Soviet party understood that the summit had been over-shadowed by Watergate... More gravely, the summit began to convince the Soviet leaders that Nixon's problems might turn out to be terminal." This perception of US weakness at the top, Kissinger has suggested, encouraged the Soviets to risk acting as boldly as they would less than four months later, when war erupted in the Middle East. The "domestic passion play," as Kissinger called Watergate, now threatened to enfeeble the nation, not only the man.

The word circulating at the New York Times was that Nixon was seeing a psychiatrist. The therapist they had in mind was Dr Hutschnecker, whom Nixon had consulted on and off since the 50s, and reporters armed with the doctor's photograph for identification began looking for him wherever Nixon went. The Washington Star-News, meanwhile, published an extraordinary, speculative piece about what would happen if a US president became mentally ill. "What happens," writer Smith Hempstone asked, "if the president becomes physically or emotionally incapacitated and is unable or unwilling to recognise that incapacity, as might well happen in the case of a mental breakdown?... That prospect is too horrible to contemplate."

According to his biographer, Senator Sam Ervin discussed just such a possibility with majority leader Mike Mansfield before they decided on the Watergate probe. It had been, even then, "that thing which was the main fear and therefore the prime issue. Which wasn't whether or not Nixon was a crook. Millions had been talking on both sides of that issue for more than a quarter century now. Everyone knew what the prime issue was. A certain thumb moving awkwardly towards a certain red button, a certain question of sanity... Query: if the man who holds the thumb over the button is mad..."

Such fears, harboured by men not known for paranoia, were now very real. "Al Haig is keeping the country together, and I am keeping the world together," Kissinger had been heard to say as summer ended. He had recently become secretary of state and, retaining his post as security adviser, now had more independence of action than ever. Nixon did not bother to attend National Security Council meetings and would reportedly often initial documents without reading them.

On October 6 came a grave military crisis. Soviet-backed Arab armies, performing militarily better than ever before, struck at Israel and for a while it seemed they might triumph. Israel was able to contain and reverse the threat thanks to a massive US airlift, but before the guns stopped firing a moment of nuclear peril would put the entire world in danger.

Nixon did not attend a single formal meeting on the conflict during the first week of the conflict. He was clear on the essentials, Kissinger recalled, not least on the fact that a massive airlift of arms supplies was essential to Israel's survival. Yet he remained "preoccupied with his domestic scandals... deflected from details". With the successful Israeli counter-attack bringing new risks and uncertainties, Kissinger, on a trip to Moscow to negotiate a ceasefire agreement, phoned the White House on an urgent matter - only to have Alexander Haig, Nixon's chief of staff, tell him, "Get off my back... I have troubles of my own."

Back in Washington, on the evening of October 24, the secretary of state found himself faced with the possibility that the Soviets might send forces to the war zone. At 7.05pm he interrupted a phone confrontation with Ambassador Dobrynin to take a call from the president. Nixon was "agitated and emotional", and his principal concern was himself. Congressional leaders, he insisted, must be told how indispensable he was to the management of the Middle East crisis. The politicians, Nixon told Kissinger, were attacking him "because of their desire to kill the president... they may succeed. I may physically die... It brings me sometimes to feel like saying the hell with it."

It was on that night that the red button moment occurred. Kissinger's conversation with Dobrynin convinced him that Moscow was intent on sending in troops. He worked the phone for two hours, then called Haig at 9.30pm, only to be told that Nixon had already "retired for the night". At a moment that he and Haig agreed might herald the presidency's "most explosive crisis", the leader of the free world was unavailable.

Roger Morris, a former Kissinger aide, has quoted the secretary of state's senior assistant, Lawrence Eagleburger, and other colleagues as saying Nixon was "upstairs drunk... slurring his words and barely roused when Haig and Kissinger tried to deal with him in the first moment of the crisis". Haig, the principal relevant witness, has said the president was merely "tired".

Minutes after his call to Haig, Kissinger heard from Dobrynin again, this time to read a peremptory warning from Brezhnev to the president. "I will say it straight," the crucial sentence read. "If you find it impossible to act jointly with us in this matter, we should be faced with the necessity urgently to consider the question of taking appropriate steps unilaterally." The letter demanded an immediate response.

Intelligence reports confirmed suspicious Soviet air and naval movements that seemed to suggest that insertion of an airborne force was imminent. The appropriate response, Kissinger reasoned, was to shock the Soviets into abandoning such plans. At 9.50pm he called Haig again. "I asked whether I should wake up the president," Kissinger recalled. "He replied curtly, 'No.' I knew what that meant. Haig thought the president too distraught to participate..."

After more phone calls and a warning to Dobrynin, the Washington Special Actions Group - made up of top defence department, intelligence and military chiefs - was summoned to the White House Situation Room. There was no vice-president to summon because Gerald Ford had yet to be confirmed by the Senate. Most US forces at that time were normally at an alert status known as DEFCON (defence condition) IV. DEFCON I is war. That night the military went to DEFCON III, or Flash III.

At US air bases, B-52s loaded with nuclear weapons lined up nose to tail. In missile silos, launch commanders buckled themselves into their chairs. Nuclear-armed submarines sped to secret positions off the Soviet coast, prepared to launch. In case those steps were not sufficient to put the Soviets on notice, two aircraft carriers were ordered to move closer to the conflict, amphibious ships were ordered to leave port in Crete, B-52s in the Pacific headed for the US, and the 82nd Airborne was placed on alert. A stern reply to Brezhnev's letter, telling him that unilateral Soviet action would, among other things, be contrary to the Agreement on Prevention of Nuclear War, went out at dawn. It was sent in Nixon's name.

A new letter from Brezhnev came that day, written as though the threat of the previous night had never been made. The crisis evaporated, and Nixon - "elated", according to Kissinger - went off to Camp David. From there he called again to ask that the press be informed how indispensable he had been.

Whether or not the president had been drunk, the fact remains that the world had been on the brink of a nuclear crisis while he was asleep. Haig, who would remain at Nixon's side until the resignation, has claimed he took the threatening Brezhnev message to the president, that Nixon called it "the most serious thing since the Cuban missile crisis" and called for action. Later, told of the decision to hold an emergency meeting, Nixon "expressed no enthusiasm for attending... As usual, he preferred to let others set the options... With a wave of the hand, he said, 'You know what I want, Al, you handle the meeting.'"

The account of another senior participant suggests that this is not the full story. Admiral Elmo Zumwalt, the chief of naval operations, recalled being summoned on the night of the alert, along with the other joint chiefs, to a 2am meeting with defence secretary James Schlesinger and his deputy, William Clements. Nixon's name was not mentioned when they discussed the alert, and Zumwalt was told that it had been initiated without the president's involvement. "We had to go on nuclear alert without his permission," the admiral recalled in 1997. "The reason we had to do that was because he could not be awakened. Nixon obviously had too much to drink... I was told at the time that they were not able to waken him."

Schlesinger is today circumspect when discussing the episode. If Nixon was unwakable, "Haig put up a good performance, going out to consult with him two or three times." Schlesinger could think of no other president of modern times who would not have been present during such a momentous meeting.

Nixon spent Christmas Day 1973 at the White House with his family, all by now in increasingly low spirits. After the holiday, Nixon and Pat headed for San Clemente, and remained there for 17 days. He had so far spent only four of the 44 weekends of his second term in Washington. To outsiders, Pat had seemed to be withstanding the turmoil better than her husband. "Amazing," a visiting African diplomat had observed. "I could see his hands shaking, and he looks grey. But she has such control."

The control had been less evident behind the scenes. According to Ehrlichman, Pat had also been drinking too much during the long crisis, and once had to be rescued by the Secret Service from an overflowing bathtub. On another night, according to an Executive Protection Service officer, agents had been obliged to rush to Pat's aid for another reason. As he allegedly had years earlier, after the 1962 defeat in California, Nixon had again struck her.

The realities - personal, political and judicial - continued to come rushing at Nixon unabated. With his staff decimated, recalled Bryce Harlow, Nixon's long-time friend and senior White House adviser, the president was "almost incommunicado. He dealt with General Haig, Ron Ziegler, Rose Woods, his counsel... stopped seeing anyone else." In part, this was because few could now bear to spend time with him, so distracted was his manner.

Many of those who had served him were already being brought to justice. Egil Krogh, sentenced for his role in the burglary of the office of Pentagon Papers leaker Daniel Ellsberg's psychiatrist, had been the first to go to jail. Nixon's attorney Herb Kalmbach would be next, convicted on a corrupt practices charge of offering an ambassadorship in return for an election contribution.

Nixon aides high and low were facing terms in jail: Charles Colson, for obstruction of justice; Jeb Magruder, for obstruction of justice and conspiracy to intercept wire communications unlawfully; John Mitchell, former Attorney General, for conspiracy to obstruct and perjury; Appointments Secretary Dwight Chapin, for lying to the grand jury; John Dean, for conspiracy to obstruct justice; Haldeman and Ehrlichman, for conspiracy to obstruct and perjury. By the end, 14 associates would serve time behind bars.

For Pat and the two girls, maintaining their composure was getting harder with every week that passed. Julie insisted in 1974 that her father "hasn't done anything wrong". By the spring, however, the strain was obvious: "She seemed emaciated, bewildered, grasping at straws," the president's fervent supporter, Rabbi Korff, wrote in a note to himself after seeing her at lunch.

"We do believe in Daddy as ever," Tricia told an interviewer in April. "He has never lied... He is the most honest man we know."

Pat, for her part, on occasion betrayed obvious distress. "Nixon came out with some Secret Servicemen," a neighbour recalled of a visit to Key Biscayne. "He was heading back to Washington, and Pat was standing there.

A tear came down her cheek, and she kind of shook her head. Nixon looked like the weight of the world was on his shoulders. He was slumping, shuffling along." Observed close up on a trip to Latin America, Pat seemed suffused with pain and rage. Asked how she was coping, she declined to reply. A happy moment, however, seemed at hand when - in Nashville en route home - she had a public reunion with her husband at the opening of the Grand Old Opry.

It was Pat's 62nd birthday, and her face glowed as Nixon took to the piano to play Happy Birthday. "At the last chord," her assistant Helen McCain Smith remembered, "Pat rose from her chair and moved toward her husband... he rose, turned, stepped brusquely to centre stage - and ignored Pat's outstretched arms. I shall never forget the expression on her face."

Anthony Lukas, one of the most reliable of Watergate chroniclers, quoted a White House aide who compared Nixon in the last days to Captain Queeg in Herman Wouk's The Caine Mutiny: "Given to sudden rages, to wild speculations, terrible doubts."

"A new element crept into our calculations about the effect our actions might have on the president," Watergate assistant prosecutor Richard Ben-Veniste remembered. "If there was a streak of instability there, then it meant we would have to be extra careful to keep from pushing Mr Nixon over some invisible line into disaster - maybe disaster for all of us."

By the summer of that last year, the apprehension had spread to Congress, focusing on two issues in particular. Was the president so disturbed that he might start a war? Also, might he attempt to use the army to seal off the Capitol or surround the White House? Senator Alan Cranston, from California, had become alarmed when, as the impeachment process got underway, Nixon began courting members of the House, inviting them on to his yacht. The president had spoken, Cranston heard, of how he could press a button and in 20 minutes, 50 million Russians would be dead, and - after that - how many Americans?

As early as June, when Nixon was on a visit to the Middle East, Gerald Ford's aides had begun considering how a transfer of power might work - or not work. Ford associate Philip Buchen, and Clay Whitehead, director of the White House's Office of Telecommunications - fearful of electronic eavesdropping - held one such conversation in a car to ensure privacy. What if Nixon was impeached and convicted in the Senate but refused to give up power? What if he lost his mind and tried to use the military to stay in office?

Friday, August 9, 1974. Morning found Nixon in the Lincoln Sitting Room, the memoirs of past presidents piled in front of him. Haig brought him a letter addressed to Henry Kissinger. The political death warrant, requiring signature.

Dear Mr Secretary,

I hereby resign the Office of President of the United States.

Sincerely,

Richard Nixon