Duncan fought during the Battle of Chapultepec on September 13, 1846, and during the subsequent capture of the Belén and San Cosme gates on September 13–14. During the occupation of the Mexican capital, a quarrel occurred between the American generals. Gideon Pillow, one of the U.S. division commanders, published an article that appeared in the New Orleans "Delta" and "Picayune" newspapers under the pseudonym "Leonidas". The letter claimed Pillow was responsible for the American victories while Scott was mostly a bystander. Everyone immediately guessed that Pillow was the author. On October 23, the Pittsburgh "Post" published a letter claiming that Worth and Duncan saved Scott from a blunder by convincing the commander-in-chief to use the southern approach to Mexico City. Actually, Duncan wrote a letter to a friend and this was the basis for the Pittsburgh letter. Duncan later took credit for the letter. This was followed by a second letter praising Pillow in glowing terms. At first, Scott tolerated Pillow's unruly behavior, but he finally had enough and brought charges of insubordination against Pillow, Worth, and Duncan.