Total percentage of expenditure for Iraq according to you is 18%. The total cost of WWII, a war fought on a global scale is 238%. So, this tiny little war is 1/15 the scale of a global war. How can that be justified?
I don't know that is can be justified, but the reasons are pretty clear.
Compare the cost of providing and maintaining WWII military equipment to that of today.
Compare what we pay each soldier to what we paid in 1945. Volunteer armies are a lot more expensive per soldier than conscript armies.
Essentially we are spending a LOT of money in a largely successful effort to reduce the number of deaths and injuries suffered by our troops.
You're alive and you are free in the face of enemies who would deny you either...or both.