The facts on the ground of German occupation in 1945-46 are nearly identical in all respects to Iraqi occupation in 1003-2004. The number of Americans involved, the casualty rates, the rates of sabotage, the rates of attacks on "collaborators" are all statistical matches between Germany and Iraq.
You are correct that Japan does not apply. However, that is because they revered the Emperor, and McArthur (wisely) left the Emperor in place and the Emperor (wisely) urged no resistance to the occupation. Your question about Italy shows you've missed a few historical points. Much of Italy fell, in the last days, to Italian partisans, rather than Allied troops. It was Italians who hung Mussolini's body from a lamppost in Milan.
America has not attempted any "nation-building" in the last twenty years that was remotely comparable to Iraq, today. But it did do precisely that, under nearly identical circumstances, in Germany in 1945-46. You need to read some more history books from WW II.