Even if there were a case for staying the current course in Iraq, America’s badly overstretched Army cannot sustain present force levels much longer without long-term damage. And that could undermine the credibility of American foreign policy for years to come. The Army has been kept on short rations of troops and equipment for years by a Pentagon more intent on stockpiling futuristic weapons than fighting today’s wars. Now it is pushing up against the limits of hard arithmetic. Senior generals are warning that the Bush administration may have to break its word and again use National Guard units to...