Prove that their presence there is unnecessary.
That is pretty common sense-the big problems in Iraq right now from the US perspective are radical groups opposed to US presence killing anything that moves. They shell the Green Zone regularly. It doesn't take any imagination or expertise to understand how useless state department personnel are against that sort of threat.
On the contrary, I clearly remember a lot of complaints in previous years that, if anything, the DoS dropped the ball since day one by being understaffed in Iraq. The military instead are made to do stuff they are not trained to do - negotiate with factions and local authorities etc. - which should be handled by diplomats.
You can't negotiate without a modicum of security for the negotiators-anyone who meets with US personnel simply gets killed. This is a pattern in Iraq, and the insurgents have proven capable of hitting people even inside the green zone. So whatever might have been true in 2003 about increased State Department presence, it clearly is not true today.
When an Iraqi warlord with his own personal army who survived the entire occupation, under Saddam, and the Iran war, can be killed within a week of meeting with President Bush, it's safe to say that the security situation is such that more diplomats are not going to achieve anything.