Operation Iraqi Freedom overthrew Saddam Hussein’s regime, but Iraq remains unstable because of Sunni Arab resentment and a related insurgency, compounded by Sunni-Shiite violence that a January 2007 national intelligence estimate says has key elements of a “civil war.” Mounting U.S. casualties and financial costs — without clear signs of security progress — have intensified a debate within the United States over whether to wind down U.S. involvement without completely accomplishing initial U.S. goals.