The American Enterprise Institute's Frederick Kagan on...

our responsibility in Iraq:

Both honor and our vital national interest require establishing conditions in Iraq that will allow the government to consolidate and maintain civil peace and good governance. It doesn't matter how many "trained and ready" Iraqi soldiers there are, nor how many provinces are nominally under Iraqi control. If America withdraws its forces before setting the conditions for the success of the Iraqi government, we will have failed in our mission and been defeated in the eyes of our enemies.

and why we need a larger Army/Marine Corps:

The strain on the soldiers and Marines must be eased. Recruiting and training takes time, of course, and many will argue that it is too late: We'll be out of Iraq before they take the field. That same argument was made in 2003, 2001, 1999, and 1997. If we'd started at any of those times to increase the size of the ground forces, new soldiers would be on the ground today where they are badly needed. How many times are we going to repeat this mistake?