The short version is that the US did a massive wargame in the Mediterranean/Middle East in 2002. A retired Marine Lt. General ran the opposing force, and consistently wrecked the "blue" team (i.e. actual US forces). Eventually, the US did the usual thing and added increasingly unrealistic constraints on the opposing force (op-for). The original op-for commander quit, and the replacement continued to do quite well.
The exercise was called a success, and the US invasion of Iraq happened a year later.
It's also discussed in an episode (including the author of the article) of Lions Led by Donkeys, an excellent military history podcast.