The president was forced into the decision in order to end a political crisis and street agitation threatening to destabilise his government, unnerving US and British allies who fear any slide into chaos could increase the threat from Islamist militancy in nuclear-armed Pakistan.

The Chaudhry affair is a long and complex story; here is an explanation:

WHO IS CHAUDHRY?

The son of a policeman, Chaudhry, 60, became a Supreme Court judge in 2000 and was appointed the youngest ever chief justice in June 2005.

He became a symbol of resistance to General Pervez Musharraf after refusing to quit in the face of pressure from the then president and his intelligence chiefs.

Musharraf suspended Chaudhry on March 9, 2007, after the judge refused to go quietly when accused of abusing his position, and various other misdemeanours.

A panel of judges was established to look into the accusations against Chaudhry, who appointed Aitzaz Ahsan, a parliamentarian and former minister from Benazir Bhutto's Pakistan People's Party (PPP), to lead his defence team.

WHY DID MUSHARRAF WANT TO GET RID OF HIM?

There has been speculation of several reasons.

At the time Chaudhry was suspended he was hearing a case he had taken up on his own volition into the disappearance of people picked up by security agencies on suspicion of being involved in terrorism. The agencies were nonplussed when Chaudhry told them to produce missing people in court.

Chaudhry had irked Prime Minister Shaukat Aziz by taking up a case looking into the privatisation of a steel firm, and cancelling the sale. He also reportedly upset influential people by taking up a case involving a land development scheme.

But the reason most people thought Musharraf moved against the judge was that the general anticipated Chaudhry would try to make him stand by a constitutional requirement to give up his role as army chief if he was to seek another term as president.

SO WHAT HAPPENED?

Protesting lawyers, led by Ahsan, held rallies to demand the independence of the judiciary. The PPP and Nawaz Sharif's Pakistan Muslim League both got behind Chaudhry's cause.

In July 2007, Supreme Court judges delivered the first ever finding against a military ruler by lifting Musharraf's suspension of Chaudhry.

When Musharraf engineered his re-election by a subservient parliament in October without first stepping down as army chief, the Supreme Court allowed the vote to go ahead while it deliberated whether the constitution had been violated and the result should stand.

Musharraf's patience snapped a month later when he declared emergency rule and dismissed judges who refused to take a fresh oath of office. Chaudhry refused, saying Musharraf's acts were unconstitutional. Having secured the presidency, Musharraf then stepped down as army chief.

WHY WAS ZARDARI RELUCTANT TO BRING CHAUDHRY BACK?

In a general election in February 2008, the PPP swept to power riding a sympathy wave after Bhutto's assassination a few weeks earlier.

Zardari took over leadership of the PPP and enticed Sharif's PML-N to join a coalition by promising to restore Chaudhry and other judges sacked by Musharraf, who was still president. It soon became apparent to Sharif that Zardari was going back on his word, and he pulled the PML-N out of the coalition.

Zardari reinstated most other judges Musharraf had dismissed but he feared if Chaudhry was restored he would rule Musharraf an illegal president and overturn an amnesty the general had given Zardari and his wife in late 2007 to allow them to return to Pakistan without fear of prosecution on old corruption cases they said were politically motivated.

In August, Zardari forced Musharraf to quit gracefully rather than face impeachment. Musharraf still needs some form of indemnity from parliament to be sure he isn't tried for violating the constitution. Sharif, overthrown as prime minister by Musharraf in 1999, has wanted Musharraf to stand trial.

Sharif last month put his weight behind plans by the lawyers for a "long march" protest across the country to press again for Chaudhry's reinstatement.

Faced by the challenge, Zardari responded by ejecting Sharif's party from power in Punjab province, after the Supreme Court had barred Sharif and his young brother, Shahbaz, from holding elected office.

That action incited the Sharifs to go with full force behind the lawyer's march, raising fear a violent confrontation would occur, and the army would be forced to intervene just a year after a civilian government was formed.