[55] In the meantime, he served on the court-martial of Fitz John Porter for his tardiness at the Second Battle of Bull Run.[56] He was convinced of Porter's guilt and voted with his fellow generals to convict Porter.[56] The trial lasted almost two months, from November 1862 to January 1863, and, by its end, Garfield had procured an assignment as chief of staff to Major General William S. Rosecrans.[57]

Chief of staff for Rosecrans[edit]
General William S. Rosecrans
Generals' chiefs of staff were usually more junior officers, but Garfield's influence with Rosecrans was greater than usual, with duties extending beyond communication of orders to actual management of his Army of the Cumberland.[58] Rosecrans had a voracious appetite for conversation, especially when unable to sleep; in Garfield, he found "the first well read person in the Army" and the ideal candidate for discussions that ran deep into the night.[59] They discussed everything, especially religion, and the two became close despite Garfield's being 12 years his junior. Rosecrans, who had converted from Methodism to Roman Catholicism, softened Garfield's view of his faith.