Comparing the extended US presence in Germany and Japan with a similar potential presence in Iraq misses an important point: Germany and Japan were aggressors who were defeated. Post-war US military presence was part and parcel of the surrender agreement. It's something the Germans and Japanese have had to put up with as a consequence of losing the war.

Iraq was invaded for the specific purpose of deposing Saddam Hussein. Whether that goal was sufficient justification for the invasion or not, the US was the aggressor, not the attacked party. So any continued US presence, no matter how benign, will always be seen by some Iraqis as a continuation of the unwarranted invasion and occupation of their country by a foreign aggressor. It's possible that the majority of Iraqis may accept a continued US presence as necessary or even beneficial, but there will be a substantial number who will see it as unjustified and will fight it tooth and nail.

I'm not saying a continued presence is or isn't necessary, only that anyone who thinks it would be accepted as freely as it has been by Japan or Germany is mistakenly looking at two radically different sets of circumstances and naively expecting the same outcome.