The danger lies in not recognising that terrorism is encouraged by weakness, not strength. Al Qaeda attacked America because it perceived the west was decadent and so assumed it was not prepared to fight. It made a big mistake over America, but it got Europe (with the exception of Tony Blair over Afghanistan and Iraq) dead right.
The history of modern terrorism is a history of appeasement. From the first Palestinian plane hijacking in 1968, the response of the west was to assume there were legitimate grievances that had to be addressed. From that point, terrorists had every incentive to continue.
Read it all, and you will understand why pulling our troops from Iraq is not an option, and why negotiations with terrorists (such as is going on in Fallujah with the Iranian puppet Al-Sadr) will only lead to further bloodshed.