On Monday president George Bush spared his close political ally Lewis 'Scooter' Libby from prison, commuting a two-anda-half-year sentence for lying to a Grand Jury in a case that involved the revealing of a CIA agent's identity. Bush's action fell short of the pardon being demanded by Libby's powerful friends . . . there were hints yesterday that this will eventually come . . .

and he continues to be subject to probation.

Why is Libby so important to Bush?

This case went to the heart of the big lie that lay behind George Bush and Tony Blair's stated reasons for going to war against Iraq: that Saddam Hussein was trying to acquire weapons of mass destruction. In his 2003 State of the Union speech, George Bush said, "The British government has learned that Saddam Hussein recently sought significant quantities of uranium from Africa." Condoleezza Rice went further to say Saddam was trying to turn "a smoking gun into a mushroom cloud".

When that canard was exposed . . . by Joseph Wilson, the man sent to investigate alleged sales of yellowcake uranium to Iraq . . . it emerged that vice-president Dick Cheney organised a campaign to discredit Wilson.

And Libby, who was Cheney's chief-of-staff, lied to protect his boss. They had it put about that Wilson's wife, the CIA agent Valerie Plame, had sent Wilson on a junket to Niger. But the leaking of Plame's name was a federal crime whose trail ended with Libby's conviction earlier this year.

What's the reaction to Bush's decision?

Outrage from Democrats and quiet gloating from Republicans. Within minutes of the announcement, rightwing observers who had blown up when Paris Hilton got a few days knocked off her jail time were saying that putting Libby on probation was enough punishment. But there may be a high political price paid by Republican candidates in the 2008 presidential election. When he was running for president, Bush loved to contrast his law-abiding morality with the slipperiness of Bill Clinton who was charged with perjury and acquitted. The New York Times said the commutation "only underscored the way this president is tough on crime when it's committed by common folk."

How does the president come to wield such power?

According to the constitution, the president "shall have power to grant reprieves and pardons for offences against the US, except in cases of impeachment". The White House receives about 600 pardon or clemency requests a year, around 10% of which are granted. Pardons were controversial from the outset more than 200 years ago because they were seen as a throwback to royal abuses of the pardon power in Europe.

What is Bush's overall pardoning/ commuting record?

As governor of Texas, Bush made a virtue of refusing pardons. Despite a phone call from the pope he allowed the first execution of a woman since the American Civil War. And he joked about how the killer Karla Faye Tucker had become a bornagain Christian on death row. In the White House he has been extremely parsimonious with his pardons, issuing fewer than any president since the second world war.

He has pardoned just over 100 people and commuted four sentences in his six-anda-half years in office.

Is there lobbying for pardons?

There is intense lobbying behind the scenes. One example involved the Israeli prime minister Ehud Barak, and many other international luminaries, who successfully pleaded with Clinton to pardon the billionaire tax fugitive Marc Rich.

Republicans sought a pardon for those involved in the Iran-Contra affair . . . and sure enough most had their convictions wiped from the record books.

How did pardoning begin?

George Washington commuted the death sentences of those involved in the Whiskey Rebellion of 1791. Appalachian settlers, most of whom were of Scottish or Irish descent, rose up to protest the imposition of heavy taxes on their moonshine. After suppressing the rebels with a militia, Washington pardoned them on the grounds that one was a "simpleton" and the other "insane".

Which presidents have exercised this prerogative most vigorously?

Clinton is the all-time champion pardoner.

In his eight years in office he issued 457 pardons, a number of them deeply controversial. Bush's father issued 77 in four years, including the officials in the Iran-Contra affair. Ronald Reagan issued 406 in eight years and Jimmy Carter 563 in four years. The most pardons and commutations . . . 2,031 . . . were issued by Harry S Truman, who served almost eight years.

Controversy still lingers over the Clinton pardon of Marc Rich, especially when it became known that he had been a middleman for highly suspicious deals with Iraq involving over four million barrels of oil.

Clinton also pardoned his half-brother Roger's conviction on drug charges . . . long after he had served his sentence.

Don't these actions undermine the authority of the judiciary?

That is the argument made by prosecutors and judges and it was made again yesterday by the special prosecutor who brought the case against Libby. When Gerald Ford pardoned Richard Nixon on 8 September 1974 for the official misconduct which gave rise to the Watergate scandal, there was outrage. Most Americans disapproved and Ford lost the presidency two years later.

Carter's grant of amnesty to Vietnamera draft evaders was equally controversial, as was the pardoning of six Reagan administration officials accused and/or convicted in connection with the IranContra affair. Clinton's pardons of six convicted Puerto Rican terrorists who had been involved in more than 100 bombings in New York City was also seen as undermining the courts . . . even though they had already served lengthy prison terms.

The recommendation is that anyone requesting a pardon must wait five years after conviction or release prior to receiving a pardon. However, as George Bush has demonstrated, a presidential commutation or pardon may be granted at any time. When Ford pardoned Nixon, the former president had never been convicted or even formally charged with a crime.