He went in expecting to find Lee the better general, but his research instead found Lee terribly mismanaged logistics, politics, and the overall command structure of the Confederate Army. For example, in 1865 the Confederates had a huge ration stockpile in Richmond that Lee, who had been given near-dictatorial power by this point, had failed to have allocated to the army before he was cut off. His army was starving and they were sitting on rations because Lee failed to manage the logistics.
"First published fifty years ago, this analysis of the generalship of Ulysses S. Grant and Robert E. Lee remains one of the most readable histories of the Civil War. The author began his research sharing the generally held assumption that Lee was a great general and Grant a clumsy 'butcher.' By the time Fuller completed his project, however, he regarded Grant as the greatest general of his age and one of the greatest strategists of any age. Grant and Lee is a compelling study not only of two remarkable men but also of the nature of leadership and command in wartime."
Was that from J.F.C Fuller, or someone else?