From Captains Quarters:

If it appears rather backwards to have Congress ignore the advice of the military commander on the ground in a war, along with the ambassador and the commander's #2, now we know why the founders made sure that the prosecution of war remained the responsibility of the executive. Congress insisted on benchmarks as their own standard of progress, and the truth is that they did a poor job of selecting them. Confronted with that truth, they have chosen to ignore the men closest to the situation and best able to analyze it in favor of their own flawed presumptions.