The Americans won the battle and Pierce helped negotiate an armistice. He then returned to command and led his brigade throughout the rest of the campaign, eventually taking part in the capture of Mexico City in mid-September, although his brigade was held in reserve for much of the battle. For much of the Mexico City battle, he was in the sick tent, plagued with acute diarrhea. Pierce charged Treasury Secretary James Guthrie with reforming the Treasury, which was inefficiently managed and had many unsettled accounts. Guthrie increased oversight of Treasury employees and tariff collectors, many of whom were withholding money from the government. The greatest challenge to the country's equilibrium during the Pierce administration was the passage of the Kansas–Nebraska Act.