The common defense  

The interim constitution is signed, but if you read the fine print, it's clear who will be in charge. It's pretty hard to think of a government as sovereign if it can't even command its own military. Article 59 (B):

Consistent with Iraq’s status as a sovereign state, and with its desire to join other nations in helping to maintain peace and security and fight terrorism during the transitional period, the Iraqi Armed Forces will be a principal partner in the multi-national force operating in Iraq under unified command pursuant to the provisions of United Nations Security Council Resolution 1511 (2003) and any subsequent relevant resolutions. This arrangement shall last until the ratification of a permanent constitution and the election of a new government pursuant to that new constitution.