On the afternoon of July 16 McDowells Grand Army, about 30,000 strong and composed, for the most part of three months volunteers supported by 1600 regulars, marched to the front and on the 18th occupied Centreville. No living American general had ever commanded so large a body of men, and McDowells experience as staff-officer in Mexico had been with a much smaller number. Excepting the regulars, the troops were raw as were likewise most of their officers; and this march of twenty-seven miles, which a year later would have been considered a bagatelle, was now a mighty undertaking. There was lack of discipline, wrote William T. Sherman, who commanded a brigade; with all my personal efforts, I could not prevent the men from straggling for water, blackberries or anything on the way they fancied. The troops did not know how to take care of their rations, to make them last the time they should, reported McDowell; moreover their excitement found vent in burning and pillaging. These excesses, however, were checked by McDowell.
Johnston, having received a telegram from Richmond to join Beauregard if practicable, managed to elude Patterson and started for Bull Run at noon of July 18. The discouragements of that days march to one accustomed, like myself, he wrote, to the steady gait of regular soldiers is indescribable. Because of frequent and unreasonable delays and lack of discipline he despaired of reaching Beauregard in time. He accordingly made arrangements for covering the final stage by rail. After a march of twenty-three miles, he and his infantry completed the remaining thirty-four by train; the cavalry and artillery continued on the wagon-road; on Saturday the 20th he had 6000 in union with Beauregard.