For the two most powerful Americans in Iraq , Gen. George W. Casey Jr. and Ambassador Zalmay Khalilzad , as for the Iraqi dignitaries who had gathered here , it was a symbolic moment : a ceremony on a bluff high above the Tigris River at which the Americans formally returned the largest of Saddam Hussein 's palace complexes to Iraqi sovereign control , 31 months after invading troops had seized it for use as an American base .