Okay, let's break this down properly then.
First off, it's very important to remember that with all three of these countries, there was always an outside threat to keep in check. We never had to be a brute and threaten "wondrous destructive violence"; West Germany never would of survived without the Allies backing them (Soviets would of loved to finish them off), the South Koreans wouldn't be around today if the US hadn't kicked the North back across the 38th Parallel, and then you have Japan who low and behold are looking at a hostile China who they had just murdered 15 million people... so they could also use a little US made firepower. In Iraq, we never had this option because the greatest threat to the Iraqi people, were the Iraqi people.
Also, it was only a matter of time until Maliki joined up with Iran and became the asshole we all knew he was capable of being. The US was never going to offer him the level of support that Iran was more than willing to make. Lest we forget where Maliki hid out for all those years *cough* Iran *cough*. Unless we were willing to use the forces in Iraq to enforce the peace, for example, deploying into Sunni areas to ensure that Iraqi government played fair, then it was never going to hold. And say what you want about Fascism, it isn't the same as the problem that Islam is today. Never underestimate the religious animosities that are at the route of these problems, and we never had to deal with anything like that. Well, I guess there was Japan but, one picture and we showed the Japanese who's the boss:
There's one final piece to the puzzle that was in play in Japan and Germany that we never got with Iraq... they were never broken. After WW2, Germany was utterly shattered; they had last a second straight war, lost millions of people, their cities lay in complete ruin... they were a people that were tired of war. With Japan, we didn't have to break the will of the entire people, but that of only the one man that mattered. And after killing around 500,000 Japanese in two devastating attacks, not to mention a third earlier in the year that burned Tokyo to the ground, Hirohito had seen enough. In both these cases, there was no long the will the fight. The Iraqis have clearly demonstrated that even after 10 years of conflict, the thirst for blood hasn't been satiated.
That small presence, would of have to of been substantial and you know it as well as I do. And they wouldn't of had the authority to curb the Iraqi Government's aggression against the sunnis because Maliki would of never of given it. Finally, it's not the aggressive neighbors that you have to worry about in this case (which was the case for the other three countries), but the crisis within. ISIS is a two bit player right now if Iraq was run by either Saddam or the US, because we wouldn't of let the Sunnis get the short end of the stick...