 Buchanan, by then nicknamed "Old Buck" by his men, commanded the 1st Brigade, 2nd Division, V Corps during the Maryland Campaign (part of Brigadier General George Sykes's Regulars).At Antietam, Buchanan strongly protested a decision to halt his advance on what he maintained was a weakly defended portion of the enemy line.In his opinion, his Regulars could have and should have carried Cemetery Hill, defended primarily by artillery with only the depleted Virginia brigade of Richard B. Garnett in support.