US soldiers took to Iraq not only war and pain, or peace and freedom, as you prefer to conceive the situation, but also their cultures. Graffiti as art or vandalism is natural part of contemporary US lifestyle. Different kind of inscriptions colour, vandalize and communicate on walls, tanks, jeeps, rocks.
An example of soldier cultivating the hobby of graffiti writing can be spotted in Shef of BK crew.
In conclusion, graffiti peacefully coexist with army but they are void. I firmly prefer graffiti dedicated to war instead of graffiti in war.