In a lengthy news conference Thursday morning, the senior U.S. military commander and diplomat in Iraq told reporters that they share cautious optimism about progress toward security and stability in the war-torm country, but they remain frustrated by the pace and lack of predictability of that progress.

Special War Coverage

Ambassador Ryan Crocker and Gen. David Petraeus spoke ahead of a statement by President George W. Bush in which he ordered no additional troop drawdowns beyond July. That leaves open the possibility that about 140,000 U.S. servicemen and servicewomen will still be in the war zone when the next president takes office.

Bush also announced shorter combat tours, but troops already in Iraq won't be going home any earlier, at least for now.

Senior defense officials said Bush would order Army units heading to Iraq after Aug. 1 to serve 12-month tours rather than their current 15-month deployment -- a move that war critics say the president had to make to ease strain on the Army.

The process of withdrawing troops is like "battlefield geometry," Petraeus said. Commanders must position forces relative to the strength of the enemy and the capabilities of the Iraqi security forces.

"That process is ongoing, and we'll continue that process in the fall," he said. "We're keenly aware of the strain on the force, of the strain on our soldiers, sailors, airmen and Marines, and civilians, and their families, and also the strain on the budget -- the sheer cost in blood and treasure of this endeavor."

"The overall development of Iraq into a stable, secure democratic state governed by the rule of law is going to be a multi-year project. There's no question about it," Crocker said. "That does not mean that over that period we are going to need to be in Iraq in anything like the numbers we are now or anything like the missions we have now, but total recovery for Iraq is going to be a very long-term affair."

Iraqi forces will need U.S. support in some key areas, such as air support, for a long time because these capabilities take time to develop, Petraeus said. He noted that until now, the focus has been solely on the counterinsurgency effort, and now the focus is shifting to building internal capabilities.

There are now more than 500,000 members in the Iraqi security forces, Petraeus said, and the United States has been steadily handing over security responsibility to them. That process has not always gone smoothly, he admitted, but the Iraqis are steadily increasing their capabilities.