I believe the initial intention was to establish a self substaining Democracy, an Iraqi army and police force capable of providing sufficient security for the populace from both interal and external threats.
All this was to happen within 2 1/2 years of the invasion. By this time (now) we would have a permanent base established with a contingent of only a 30-40000 soldiers deployed similar to the what we have in Korea.
The benefits of a permanent base in a stable moderately friendly oil rich ME country that happen to border Iran are obvious.
Who said it was supposed to happen in 2 1/2 years?