Of course it’s about oil (Jim Holt in the London Review of Books):

It has been estim­ated, by the Council on Foreign Relations, that Iraq may have a fur­ther 220 bil­lion bar­rels of undis­covered oil; anoth­er study puts the fig­ure at 300 bil­lion. If these estim­ates are any­where close to the mark, US forces are now sit­ting on one quarter of the world’s oil resources. The value of Iraqi oil, largely light crude with low pro­duc­tion costs, would be of the order of $30 tril­lion at today’s prices. For pur­poses of com­par­is­on, the pro­jec­ted total cost of the US invasion/occupation is around $1 trillion.