The Supreme Court decision was the pivot. Of all the cases argued by his lawyers since he took office, this was the one most keenly fought and the defeat most bitterly regretted.
A woman named Paula Jones had alleged that Mr Clinton called her to a hotel room in the Arkansas capital, Little Rock, in 1991, when he was state governor and she a state employee, and solicited her for oral sex. Three years later, when he was President, she brought a case for sexual harassment and defamation, following the appearance of a magazine article naming a certain "Paula".
Mr Clinton's lawyers argued that the case should be deferred until he had completed his term of office, arguing that the case would contravene the principle of separation of powers between the executive and the judiciary and distract the President from his duties. The Supreme Court ruled unanimously that the President was not above the law and that Ms Jones's rights should not be infringed just because the individual she was suing happened to have been elected President in the interim.
A very few legal experts disputed the ruling from the start, mostly on the grounds of distraction. At each stage of the Lewinsky case, they have pronounced themselves vindicated. Most, however, concurred with the court that a president was as answerable for his personal conduct as anyone else. That view may in future be contested - quoting the implications of Jones v Clinton. But it will not help Mr Clinton. If the Paula Jones suit had not proceeded, it is at least arguable that Mr Clinton's relationship with Ms Lewinsky would never have seen the light of day, at least not as a judicial matter. No court would have had occasion to assemble women from Mr Clinton's past, as Ms Jones's lawyers did in an attempt to prove a "pattern of behaviour" on his part. Mr Clinton would not have been required to testify under oath about his sex life, and Ms Lewinsky would have remained in obscurity. Neither would have misrepresented their relationship under oath; Mr Clinton would have had no need to try to tailor other witnesses' accounts - if he did.
Mr Clinton might still have found himself in difficulty from the combination of Ms Lewinsky's resentment at the ending of the relationship and the tape-recordings made secretly by her then colleague, Linda Tripp. These could have brought the relationship into the open and caused Mr Clinton grave political damage, especially as Ms Lewinsky had proof of the relationship in the now famous "semen-stained dress". The judicial aspect, though, would have been absent. Mr Clinton would have been judged in the court of public opinion, and may well have fared better.
A more complex question is whether Mr Clinton's decision first to deny and then to dissemble his relationship with Ms Lewinsky reflected his fear of the Paula Jones suit or his fear of personal and political embarrassment. Nothing that he has said suggests that his denial was designed primarily to affect the Jones case; rather the reverse. His chief fear seems to have been that the Paula Jones case could uncover an affair he desperately wanted to conceal.
Many have argued that if Mr Clinton had come clean in January with the sort of abject confession and plea for forgiveness that he offered at Friday's White House prayer breakfast, he would have disarmed his accusers and the Lewinsky case would have been no threat. Without the Paula Jones case, that is probably true. With the Jones case, however, the judicial aspect remains.
If, on the other hand, Mr Clinton had given a full and truthful account in that case, he would have been free of any perjury accusations and might have escaped accusations of witness-tampering. But he might still have been suspected of abuse of power if he tried to buy Ms Lewinsky's silence by arranging a well-paid job.
Even with the Paula Jones case proceeding and his own decision to conceal the relationship with Ms Lewinsky, the President might still have been in the clear, had Ms Lewinsky not agreed to change her testimony. That is what suddenly speeded up the case through August to this month's frenetic climax. If she had stuck to the denial she agreed with Mr Clinton in advance, the existence of the relationship would have been much harder for a court to prove. To judge by the Starr report, however, the circumstantial evidence was so strong, that - combined with Ms Tripp's tapes - it would have been persuasive.
Ultimately, Mr Clinton has a much earlier decision, which was entirely his own, to blame for his predicament: his decision to embark on, and then continue, a relationship with a White House trainee that broke many of the taboos of Nineties America: sex in the workplace, sex with an employee, sex with a far more junior colleague. Even if a young woman had walked into his office stark naked and pleaded "take me", commented one hardline (female) lawyer, Mr Clinton had a duty, as President, to send her smartly away, calling his security staff if necessary.