The troops were armed with the best weapons obtainable, the artillery was numerous and excellent, the army was well fed, and so confident were the men of success that they regarded the whole affair in the light of a great picnic.

The grand army numbered 55,000 men, with nine regiments of cavalry and forty-nine rifle-guns.

To oppose these, the Confederate force, after the arrival of Johnston's army, numbered 27,833 infantry, thirty-five smooth-bore guns, and 500 cavalry.

Many of the infantry were armed only with shot-guns and old fowling-pieces, and the guns were small and ill-supplied with ammunition.

There had been some sharp fighting on the 18th, and the Federal advance across the river of Bull Run had been sharply repulsed, therefore their generals determined, instead of making a direct attack on the 31st against the Confederate position, to take a wide sweep round, cross the river higher up, and falling upon the Confederate left flank, to crumple it up.
All night the Federal troops had marched, and at daybreak on the 21st nearly 40,000 men were in position on the left flank of the Confederates.