What we have observed since the fall of Baghdad:
The main events, in short had been Osama bin Laden seeing an
opportunity to use Iraq as a new terrorist base since it had fallen
into anarchy and was occupied by a hated army (that of the US).
Enter Al-Zarqawi following Saddam's capture. Basically, Al-Zarqawi
is playing up the sentiments of the disenfranchised Sunnis and
turning them over to Osama's cause.
If the US had not decided to take over the oil supply in Iraq,
maybe there would be less reason for Al-Qaeda to set their sights
on Iraq. Saddam was a dictator indeed, but did he really have any
links to terrorist groups? The US intelligence appeared dodgy at
times. Hence there was a lack of support from key NATO allies like
France and Germany. If you have also noticed, there had been no
terrorist attacks on French or German soil for quite a while.