just my opinion:
the time we wasted prior to our operations in Iraq - the 9 months or so we played patsies with the UN - the baathists and al quaeda operatives stashed arms and increased existing, prepositioned caches for such operations.
stuff was moved out of Iraq, cells were dispersed and whoever survived the initial contact with us cooled it for a short while and then came out blazing. operatives from Iran, Syria, Chechnya, etc. all rallied to the call for jihad.
well trained cells don't need large command structures to operate and can be very successful in just this type of scenario. hit and run - there are al-zarqawi's out there and probably a leader in the area but the suicide bombers can be a prepositioned one man show.
its pretty much a given some iranian communication net exists, and syrian as well, to keep fresh fighters coming in and direct in a very general way. again, it does not take thousands of men enmasse to do a lot of damage. just a few well placed fanatics.