Rudy Giuliani's place in history was made on 11 September 2001, when the Twin Towers of New York City's World Trade Center were brought down by a pair of hijacked airliners.
Mr Giuliani was mayor of New York at the time, and immediately won praise for his determined response to the tragedy.
On the strength of his heroic reputation, "America's Mayor" - as admirers called him - decided to seek the Republican Party's nomination for the 2008 presidential election.
But he dropped out of the race when his gamble to focus on big-prize states failed to pay off, wasting an early lead in the polls.
Mr Giuliani, a native New Yorker, was born in Brooklyn in 1944, the grandson of Italian immigrants.
He began his career as a lawyer, working mostly in government.
He was a top Justice Department official under Ronald Reagan in the early 1980s, before returning to New York as a public prosecutor.
He targeted top Mafia figures - including the heads of New York's so-called Five Families, whom Mr Giuliani said he intended to "wipe out" - and corrupt Wall Street traders Ivan Boesky and Michael Milken.
Born 28 May 1944 in Brooklyn, New York
US Attorney for the Southern District of New York, 1983-1989
Mayor of New York City, 1994-2001
Time magazine Person of the Year, honorary knighthood
Mr Giuliani first ran for mayor of New York in 1989, running as a Republican in a heavily Democratic city and losing by a razor-thin margin.
He was successful on his second attempt, defeating incumbent David Dinkins in 1993.
As was perhaps to be expected given his background as a prosecutor, Mr Giuliani set out to bring New York crime under control as mayor.
He instituted controversial policies such as "zero-tolerance policing", under which even the smallest of offences were prosecuted in order to inculcate respect for the law.
At one point tens of thousands of people were suing the New York Police Department over alleged abuse by officers.
The city's crime rate did fall during his tenure, for which he takes credit. Critics say the drop in crime was due to broader social factors such as an improving economy.
He was elected to a second term in 1997, and, barred by law from running for a third term, considered running for Senate in 2000 against Hillary Clinton, then the outgoing First Lady.
A combination of factors including a messy divorce and prostate cancer led Mr Giuliani to withdraw from the Senate race before it had got far.
Mr Giuliani's final term as mayor was drawing to a close when 11 September 2001 dawned.
His leadership on the day and in the aftermath of the attacks made him a hero to many.
Time magazine named him its Man of the Year for 2001, and Queen Elizabeth II presented him with an honorary knighthood.
Of course, even then he had his doubters - including veteran New York reporters Wayne Barrett and Dan Collins, who wrote a highly critical study called Grand Illusion.
They charged that bad decision-making before 11 September led to more deaths than necessary when the city came under attack.
He was accused of racing to reopen lower Manhattan too quickly, before air quality was safe enough. Some New York firefighters have attacked his leadership too.
But such criticism did little to dent Mr Giuliani's popularity.
After leaving office in late 2001, the former mayor pursued a successful consulting career, while nurturing political ambitions.
In 2007, Mr Giuliani staked his presidential hopes on his image as a tough leader and his law-and-order conservatism - even though he is comparatively liberal on some social issues.
Throughout that year he was a front-runner for the Republican nomination in national polls.
However, his strategy of leaving campaigning in early primary states Iowa and New Hampshire largely to his rivals while focusing on big states went disastrously wrong.
He dropped out of the race after coming third in the Florida primary in late January, and endorsed his main rival, fellow Republican moderate John McCain.
It was always dubious whether social conservatives would back Mr Giuliani, given his history of supporting abortion rights and gun control, both anathema to many Republicans.
He is also on his third marriage, to a nurse with whom he began an affair while married to his second wife, a potential sticking point for social conservatives.