London has been the target of coordinated terrorist bombings for more than two decades.
As Irish republicans fighting the British army in the 1970s were quick to realise - one bomb in London was worth far more in terms of its impact and publicity than a dozen in Northern Ireland.
On mainland Britain, interest in events in Belfast and Londonderry quickly faded, but the Provisional IRA took their fight to the UK capital in an effort to shake the resolve of the public and put pressure on the government.
London was not only a potent symbol with the national media, on hand to record any devastation, but it housed a large Irish community in which IRA active service units were able to move easily.
The public outrage caused by bombing atrocities contributed to grave miscarriages of justice - the Birmingham six and the Guildford four - which were in a sense another IRA "success" in humiliating the British state.
The parallels with groups like al-Qaida, which can move within London's large Muslim communities, are obvious, and figures like London mayor Ken Livingstone today tried to stress the need for Londoners to stand together rather than turn on innocent Muslims.
Almost equally disturbing for a short time were the two nail bombs targeted at the black and Asian communities of Brixton and Brick Lane in April 1999, followed by an attack on a gay pub, the Admiral Duncan, in Soho. These were revealed to be the work of a lone hate-filled individual, David Copeland.
The IRA bomb campaign, which hit London from 1973 onwards, sought to create a climate of fear over a long period, but it soon tried to explode two or more devices at a time to maximise the havoc. There were 36 bombs in London in 1973.
Over 20 years, the IRA strategy shifted away from causing as many casualties as possible to trying to cause economic damage to London's financial centre.
The major incidents were:
1973, March 8: two IRA car bombs in London explode outside the Old Bailey and government's agriculture department headquarters, killing one person and wounding more than 150
1974, October 5: two IRA bombs explode in pubs in the London suburb of Guildford; five dead, more than 50 injured
November 21: two IRA bombs in Birmingham kill 19 and wound more than 180
1982, July 20: two IRA bombs in Hyde Park and Regent's Park in London kill 11 British soldiers and wound more than 40, mostly civilian onlookers
1983, December 17: IRA car bomb explodes outside Harrods department store, killing six people and wounding about 100
1984 October 12: IRA targets conference of ruling Conservative party, killing five and wounding 24, but narrowly missing the prime minister, Margaret Thatcher
1989, September 22: the IRA bombs the Royal Marines School of Music in Deal, killing 10 soldiers and wounding more than 30
1991, February 7: IRA fires three homemade mortar shells at No 10 Downing Street, the British prime minister's official residence in London. No injuries
1992, April 10: a massive IRA truck bomb in London's financial district kills three and causes hundreds of millions of pounds worth in damage
April 24: an IRA truck bomb in London's financial district, killing one and causing heavy damage
1996, February 9: IRA ends a 17-month ceasefire with a third massive truck bomb in London's financial district, killing two
February 18: an IRA bomber accidentally kills himself aboard a London double-decker bus, five injured
June 15: for the first time, the IRA targets a different English city - Manchester - with a massive truck bomb, wrecking the central shopping area and wounding about 200
2000, September 20: IRA dissidents fire rocket-propelled grenades at headquarters of MI6 security agency. No injuries.