A The immediate reason was a court summons, a subpoena, issued by the independent prosecutor, Kenneth Starr, four weeks ago. Mr Starr wanted to ask Mr Clinton about his relationship with Monica Lewinsky, who was one of several hundred unpaid trainees - or "interns" - working at the White House in 1995. Mr Starr is investigating allegations that Mr Clinton had an affair with her, lied about it under oath and induced her to lie about it in a sworn statement. If he can prove this, Mr Clinton risks charges of perjury and obstruction of justice, and could be impeached.
Q Did Mr Clinton have to testify?
A Technically, he could have used the courts to test whether sitting presidents are subject to subpoena, but such a challenge was not supported by leading Democrats and risked provoking a constitutional crisis by pitting two branches of US power - the executive and the judiciary - against each other. Mr Clinton avoided such a stand-off by agreeing to testify "voluntarily" on closed circuit television from the White House. The subpoena was withdrawn. Some lawyers still believe that he should not have agreed to testify.
Q But why is the President's relationship with Ms Lewinsky any of the prosecutor's business?
A This is what many Americans are asking. It all goes back to two decisions long ago. In 1994, a former Arkansas state employee, Paula Jones, belatedly decided to sue Mr Clinton in the civil court for sexual harassment, alleging that he invited her to an Arkansas hotel room when he was state governor and solicited oral sex. (She refused.) Mr Clinton's lawyers pleaded that the case should be deferred until he had left office. In May 1997, the Supreme Court ruled otherwise, saying that Ms Jones had rights, too, and the case would be a minimal distraction for the President. How wrong can the cream of US jurisprudence be?
With the case finally proceeding, Ms Jones's lawyers tried to strengthen her case by demonstrating a "pattern" of sexual misconduct by Mr Clinton, and hunted down several women from the President's past as witnesses, among them Monica Lewinsky. In the end, the case was thrown out, and the Lewinsky allegations immaterial as post-dating the alleged incident. But the damage had been done.
Both Ms Lewinsky and the President were required to testify under oath; both denied a sexual relationship. And there everything might have rested, had it not been for Linda Tripp. Ms Tripp, a friend of Ms Lewinsky's at the White House and then at the Pentagon, had taped 18 hours of conversations in which Ms Lewinsky poured out details of an 18-month affair with the President. In January 1998, learning of Ms Lewinsky's sworn denial, Ms Tripp passed the tapes to Kenneth Starr's office, which was already investigating the President, as possible evidence of a crime (perjury).
Q How come Kenneth Starr was investigating the President of the United States?
A Mr Starr had been appointed by the attorney general, Janet Reno, in 1994 to continue an existing investigation into whether fraud was involved in a failed land development scheme in Arkansas, known as Whitewater, in which the Clintons had invested. There were allegations that Mr Clinton might have used his position as state governor to cut his losses. A dozen people have now been convicted, but not the Clintons.
Q But why was the Lewinsky investigation tagged on to Whitewater?
A Because one main strand of the Whitewater case was possible abuse of power, including the allocation of well-paid sinecures, to silence key witnesses. Jobs offered to Ms Lewinsky after she left the White House appeared to follow that pattern.
Q What is a grand jury?
A The federal grand jury that will hear Mr Clinton's testimony comprises 23 individuals selected at random, under the supervision of a judge. Their hearings amount to a preliminary investigation to decide whether there is sufficient evidence to justify bringing criminal charges.
Q What is the evidence against Mr Clinton?
A This is confidential, but leaked reports attest to the existence of circumstantial evidence such as White House entry logs showing repeated visits by Ms Lewinsky after she was transferred to the Pentagon, testimony from White House staff that she hung around in the vicinity of the Oval Office, an area usually off-limits to trainees, and records of gifts they exchanged.
Until two weeks ago, Ms Tripp's tapes of Monica's confessions were the best weapon in Mr Starr's armoury, though they conflicted with her sworn denial. Then, however, Ms Lewinsky concluded a deal with the prosecutor under which she would admit a sexual relationship with the President in return for immunity from prosecution for perjury. She would also hand over a dress that was rumoured to be stained with the President's semen. Ms Lewinsky, who testified 10 days ago, and her dress could be Mr Starr's strongest suit.
Q But isn't it all just about sex?
A Many Americans believe that it is, and that it is therefore a private matter for Mr Clinton, his wife and Ms Lewinsky. What Mr Starr is investigating, though, is not so much whether there was an affair, but whether Mr Clinton perjured himself when he denied it and whether he may have tried to save himself by persuading Ms Lewinsky to lie. These, as legal experts point out, would be serious offences.
Q What has Monica said?
A Until 10 days ago, she had said absolutely nothing (except what was on the very partially released tapes), and she has still said nothing in public. On 6 August, she testified before the grand jury and, according to sketchy leaks of what she said, acknowledged "a certain sort of sexual relationship" with the President (described graphically described in Ms Tripp's tapes as oral sex and "phone sex"). She also admitted discussing with Mr Clinton strategies for concealing the affair, but denied that the President told her to lie.
Q What has Bill said?
A He has strenuously denied any sexual relationship with Ms Lewinsky and denied telling her to lie - in his sworn testimony to the Paula Jones investigation in January, in a television interview after the allegations first surfaced in January, and at a subsequent press conference where he said: "I did not have sexual relations with that woman, Ms Lewinsky." He also promised to tell the truth "sooner rather than later". Two weeks ago he promised to testify "completely and truthfully", but he has reportedly resisted pressure from advisers and leading Congressmen to "confess" as a) legally unjustified and b) politically risky. Latest reports say he may admit to oral sex, but insist this does not constitute "sexual relations".
Q What would be the best outcome for Clinton?
A That the tests on the dress are negative, or inconclusive, and the grand jury cannot decide which of Ms Lewinsky's two versions - her sworn statement or the tapes plus her testimony - to believe. It would then be a "he said/she said" scenario. Legally, he would be untouchable; politically, he would be almost unscathed. He could complete his term and maybe go down in history as an effective president, responsible for major reforms of health and welfare, and a tailoring of US policy to the global economy.
Q What would be the worst outcome?
A If the tests on the dress are positive and a DNA test proves the semen to be his, or if other evidence allows Mr Starr to build a case for perjury and/or obstruction of justice. There could then be a sordid legal argument about what constitutes "sexual relations" - women previously associated with Mr Clinton claim that he does not regard oral sex as "real" sex. Any further action would depend on the US Congress when it receives Mr Starr's report.
Q What are the options for Congress?
A The judiciary committee of the House of Representatives could receive the report as early as mid-September. It would then have to decide whether there were grounds for impeachment and, if so, forward the matter to the Senate judiciary committee for action. Congress could also decide to do nothing.
Q What are grounds for impeachment?
A The Constitution talks of "high crimes and misdemeanours". Legal experts are divided about whether perjury alone would meet the definition, especially in a civil suit. Obstruction of justice probably would, but Congressmen are answerable to their constituents and there seems little appetite in the country to impeach a popular president for what is perceived to be fundamentally a sexual indiscretion. Some have suggested that a reprimand could suffice.
Q If the report contains evidence of crime, could the President not be prosecuted?
A Whether a sitting president can be indicted or must first be stripped of the protection of his office by impeachment is a point much debated by legal minds in recent weeks.
Q Where is Hillary in all this?
A The President's wife "stood by her man" for the second time in his national political career when she went on television in January to reject the allegations against him and blame "a vast right-wing conspiracy" for his troubles. Last week, Mrs Clinton added anti-Arkansas prejudice in Washington as a factor. Some say that the Clintons' relations have cooled as more details of his relations with Monica have emerged and predict a divorce suit the moment he leaves office. Others say she is reconciled to his weaknesses and they are devoted to each other. The body language provides support for both views - but they were holding hands again last week.
Q What about that "conspiracy"? Are the allegations against Mr Clinton politically motivated?
A This is the 10,000-dollar question. Paula Jones's case has been funded by the Christian fundamentalist right. Linda Tripp has contacts on the Republican right. But there is no direct evidence of political motivation behind the Starr investigation.
Q What does the Lewinsky affair mean for Vice-President Al Gore?
A It all depends. If Mr Clinton is politically wounded, but survives, things are bad for Mr Gore. He is tarnished by association and could find it hard to win the Democratic nomination in the year 2000, let alone the presidency.
If Mr Clinton were forced out, Mr Gore automatically becomes president and so gains a head start for the year 2000, a chance to separate himself from Mr Clinton and show his mettle.
Q Why are the Republicans so quiet?
A Partly because they fear catapulting Al Gore into office and reducing their chances of the presidency next time round, partly because they believe that impeaching a popular president would rebound against them at the ballot box, and partly out of high-minded respect for the institution of the presidency. Mainly, though, because they are terrified by rumours of an "Armageddon" scenario: a White House task force is said to be compiling a dossier on leading Republicans' peccadilloes, ready for immediate detonation in an emergency.
Q And what do the great American people think about it all?
A A steady two-thirds of those polled say that they like Mr Clinton, think he is a good president, believe he lied to them about Monica - and, frankly, my dear, don't give a damn. But that may reflect their unwillingness to confront the issues. If Mr Starr can produce irrefutable proof of steamy telephone calls, nocturnal trysts in the Oval Office and semen stains on the dress, however, they could rapidly change their mind.Reuse content