This really is one of the most pig-ignorant dumb things that Rumsfeld has said.  Matt Yglesius explains:

"The U.S. military will rely primarily on Iraq’s security forces to put down a civil war in that country if one breaks out, Defense Secretary Donald H. Rumsfeld told lawmakers yesterday," thus emerges the dumbest thing I’ve heard today. Iraq’s security forces are a party to that country’s sectarian violence. A civil war "breaking out" would just be an intensification of that violence. Iraq’s security forces might well win a civil war against Sunni fighters, with or without American help, but they certainly aren’t going to prevent one.

Here’s specifically why leaving it up to the Iraqi security forces to keep civil war at bay, is a ridiculous and untenable position.  From Steven Biddle, a Senior Fellow in Defense Policy at the Council on Foreign Relations, in Foreign Affairs:

Iraq’s Sunnis perceive the "national" army and police force as a Shiite-Kurdish militia on steroids. And they have a point: in a communal conflict, the only effective units are the ones that do not intermingle communal enemies. (Because the U.S. military does not keep data on the ethnic makeup of the Iraqi forces, the number of Sunnis in these organizations is unknown and the effectiveness of mixed units cannot be established conclusively. Considerable anecdotal evidence suggests that the troops are dominated by Shiites and Kurds and that the Sunnis’ very perception that this is so, accurate or not, helps fuel the conflict….) Sunni populations are unlikely to welcome protection provided by their ethnic or sectarian rivals; to them, the defense forces look like agents of a hostile occupation. And the more threatened the Sunnis feel, the more likely they are to fight back even harder. The bigger, stronger, better trained, and better equipped the Iraqi forces become, the worse the communal tensions that underlie the whole conflict will get.