This, according to one United States general, is because most attacks are carried out by insurgent fighters who meet up in mosques for the weekly prayers on Fridays. They then plan for a day or two, before launching a strike and lying low again for the rest of the week.

The theory was first disclosed last week by Brigadier General Mark Hertling, who was speaking in the aftermath of a spree of suicide car bombings in Baghdad, and the rocket attack on the al-Rasheed hotel the day before.