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Petraeus' purvey is Iraq. It makes no sense to think that Petraeus, any time soon, would voluntarily argue that more of his troops need to be sent from Iraq to Afghanistan. This would be like asking Eisenhower if he thought it was a good idea to send some of his troops to the Pacific, or Grant in 1865 if he wanted to send some of the Army of Potomac men to Sherman out west.
A nit, but the Grant example is incorrect. As of March, 1864, Grant was promoted to lieutenant general and general-in-chief of all of the armies of the United States. General Grant would have been exactly the person to make decisions such as moving troops from the Army of the Potomac to the western theater.
Major General George Meade remained the head of the Army of the Potomac until the end of the war, but was overshadowed by Grant who attached himself to the Army of the Potomac and made the strategic decisions. But Grant was commander of all of the armies, not just the Army of the Potomac.