They are all in the same bed together and they are all out to kill us.
Syria is controlled by a Baathist regime which is feeding "insurgents" into Iraq to destablize it. So is Iran which is also trying desparately to build a nuclear bomb so they use it - on us or the small numbers of allies we have.
Whatever the reasons for going into Iraq first - and there were many - we have made a policy decision to try and set up a stable government there. Such a result will be impossible if we leave the present regimes operating in Iran and Syria.
As for Saudi Arabia, Wahhabist clerics are the backbone of anti-American, anti-western terrorist groups. And Saudi Arabia is their capital and their banker and we have allowed these demons to set up shop right here in America.
STRATEGERY! Look at a map and your first comment will make all the sense in the world, coupled with all the other reasons for going to Iraq first. IMHO