 The Union 's V Corps , commanded by Brig. Gen. Fitz John Porter , took up positions on the hill on June 30 . McClellan was not present for the initial exchanges of the battle , having boarded the ironclad USS Galena and sailed down the James River to inspect Harrison 's Landing , where he intended to locate the base for his army . Confederate preparations were hindered by several mishaps . Bad maps and faulty guides caused Confederate Maj. Gen. John Magruder to be late for the battle , an excess of caution delayed Maj. Gen. Benjamin Huger , and Maj. Gen. Stonewall Jackson had problems collecting the Confederate artillery . 
