Just back from Baghdad and eager to discuss promising developments, Secretary of State Condoleezza Rice found herself knocked off message Sunday, forced to defend prewar planning and troop levels against an unlikely critic - Colin Powell, her predecessor at the State Department. For the Bush administration, it was a rare instance of in-house dissenter going public. On Rice's mind was the political breakthrough that had brought her and Defense Secretary Donald H. Rumsfeld to Iraq last week and cleared the way for formation of a national unity government. Yet Powell sideswiped her by revisiting the question of whether the US had a large enough force to oust Saddam Hussein and then secure the peace. He said he advised Bush before the US-led invasion in March 2003 to send more troops to Iraq, but that the administration did not follow his recommendation.