Arrested a month before hijackers crashed four jetliners into New York's World Trade Centre, the Pentagon and a field in Pennsylvania, Moussaoui is symbolic of a conflict different from any the United States has ever fought.
Moussaoui is a French citizen of Moroccan decent who was the product of a broken family. His path to one of Usama bin Ladin's training camps in Afghanistan apparently began when he moved to England and became involved with radical Muslim clerics.
"In London, he was far away from me" and "I was his only safeguard," Abd Samad Musawi wrote of his younger brother in a book, "The Making of a Terrorist".
Picked up in August 2001 after arousing suspicion at a flight school in the state of Minnesota, Moussaoui was transformed from an immigration violator into a terrorist defendant three months after the 11 September, 2001, attacks.
His criminal trial was to have been an evidentiary showcase detailing the horror of al-Qaida. Those plans would be scuttled by Friday's scheduled guilty plea to a six-count indictment.
Jumping past a trial, the lawyers Moussaoui tried to fire would eventually be defending him in a penalty phase proceeding, a mini-trial or sorts before 12 jurors who will decide whether to spare his life.
But Moussaoui still could change his mind about pleading guilty, which he did once before.
Some legal experts say Moussaoui's decisions seem to make no sense, unless he wants to die.
One possibility is that "he was deprived of his martyrdom and feels the only way he can achieve that lofty state is simply to admit to the crimes," Washington defence attorney Richard Hibey suggested.
"It seems that he is using the system to make a political statement regardless of what implications it has for him"
criminal defence attorney
Criminal defence attorney David Schertler says Moussaoui "defies any conventional sense of what a defendant is and what a defendant is trying to accomplish. It seems that he is using the system to make a political statement regardless of what implications it has for him."
After meeting on Wednesday with Moussaoui, the judge ruled that he was fit to enter a plea.
Nearly 3000 people died in the attacks, and Moussaoui is charged with conspiracy to commit acts of terrorism, commit aircraft piracy, destroy aircraft, murder government employees and destroy property.
The first four charges carry a maximum sentence of death.
Two failed lawsuits in which Moussaoui sought $20 million each for alleged jailhouse abuse provide new glimpses into his thinking after three years of solitary confinement at the Alexandria, Virginia, Detention Center.
In the second case, Moussaoui apparently resisted efforts to take him to the courthouse for a scheduled deposition. Moussaoui has been difficult to handle at times, John Clark, the US marshal for the Eastern District of Virginia, said.
"He can be somewhat moody," said Clark, adding that Moussaoui's allegations were overdramatic.
Whether Moussaoui was intended to be a participant in the 11 September attacks is unclear.