What is US doing in Iraq?

As far as I remember,US wanted to invented Iraq under any cirstances.At beginning US said that Iraq has a weapons of m distraction and we should invent Iraq for that.Well,rest of the World and UN told to US,that is no such weapons over there,but any way US attacked Iraq.Shortly after that of course US never found those weapons,so they have to became with some thing.Of course "good old-bring freedom and democracy".But if US want to bring "Freedom and Democracy" why they didn't"t do this in N.Korea,Cuba,Somalia and other places all over the World?Another one is"fighting terrorism".What kind of terrorism -Ben Laden has nothing to do with Iraq....Lately I hear US wants their oil.I hope it is not true,because if it is,how can country,which choose own self to be world policeman be so unfair full to do so?If it is true,who is next S.Africa because they have diamonds,India because they have bengal tigers(joke)?Of course they not going to attack Russia or China,because Russia and China has pretty strong armies and it will be very questionable after all if US made a wright decision to attack them.Any way,what is US doing in Iraq in a first place?