As the Battle of Churubusco approached, Scott ordered Pierce to the rear to convalesce. He responded, "For God's sake, General, this is the last great battle, and I must lead my brigade." Scott yielded, and Pierce entered the fight tied to his saddle, but the pain in his leg became so great that he passed out on the field. 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 remained in command of his brigade during the three-month occupation of the city; while frustrated with the stalling of peace negotiations, he also tried to distance himself from the constant conflict between Scott and the other generals.