some say they simply do not have the desired result. Gregory Johnsen of Princeton University is an expert on Yemen and he told me that the rain of drone attacks has strengthened the hand of terrorists there.
"Look at Yemen on Christmas Day 2009, the day the so-called underwear bomber attempted to bring down a flight over Detroit.
"On that day al-Qaeda numbered about 200 to 300 individuals and they controlled no territory. Now today, two-and-a-half years later, despite all the drone strikes al-Qaeda in the Arabian Peninsula has tripled in size, it's now around 1,000 members and it controls significant territory.
"The more the US bombs, the more they grow."