So far only the perpetrators seem to know the reasoning behind the London bombings yesterday. But the main candidate explanation so far seems to be that this was intended as revenge for the war in Iraq.
If so, among the expressions of shock and accusations of barbarism, it might be well to reflect that various supposedly civilized countries have been guilty of such barbarism at times, as a matter of official policy. The indiscriminate killing of innocent civilians in the course of a conflict isn't nice; arguably it is indeed immoral and uncivilized. But, in the Second World War (for instance), the Germans did it when they bombed British cities; the British and Americans did it when they bombed German cities; and the Americans did it most memorably when they atom-bombed Japanese cities.
If Saddam Hussein had been able to mount an air strike on London, it would have counted as a normal act of war by modern standards. When a non-governmental organization such as Al Qaeda bombs cities, it's doing exactly the same thing but on a lower budget. An objective observer would condemn all such behaviour no matter who does it.
When the British government decided to go to war in Iraq, it exposed both its soldiers and its civilians to possible retaliation by the enemy. It hardly ever happens in war that all the casualties are suffered by the other side.
The most guilty person in Iraq was and is Saddam Hussein, a mass murderer and warmonger. It would have been a good result if the Americans could have eliminated him and perhaps some of his accomplices without harming anyone else and without setting foot in Iraq. Perhaps in future they'll be capable of such operations. They should have enough motivation to consider it: the Iraq operation has been a horrible mess, killing lots of innocent people, leaving the country in chaos, and spending vast amounts of money; and Saddam Hussein still sits in comfortable captivity while all this goes on.