The march to Manassas took longer than planned and it was July 21, before the federals reached Centerville, VA where the Confederates controlled the lower crossings of Bull Run Creek.
McDowell's plan was to fake an attack on the confederate positions on Bull Run with half his forces while marching the rest upstream to Sudley Springs where he would cross Bull Run and turn the flank of the Confederates. Porter's brigade was to be part of the flanking forces. Speed, timing, and coordination were needed to carry out this plan, making it a very risky move considering all the green troops making up McDowell's army.
Porter's Brigade and the Marines did their part falling into marching order at 2:00am. Despite the early start confusion reigned and the Army became hopelessly confused in the darkness. The Marines found they had another problem, even after daylight when the confusion had abated some. Griffin's Battery (consisting of 6 horse drawn cannon and often called a flying battery)did just that and raced ahead whenever possible, forcing the Marines to double-time through most of the march, exhausting them before they ever reached the battle field.
Once the Marines crossed Bull Run they were rested for half an hour while the rest of the brigade caught up. The first Union brigade to cross was under Brigadier General Ambrose Burnside and they advanced on the Confederate flank near Sudley Ford. Burnsides men engaged the enemy while Porter moved his men up on their right. Griffin's Battery pushed ahead of porter's column, followed closely by the Marines, and opened fire from a range of 1,000 yards. The six guns silenced a Confederate battery and advanced to the right as the Confederate line yielded. Again Griffin's Battery pushed forward with the Marines close behind, while Burnsides men forced the Confederates back. After several hours of combat Burnside's men began to tire and withdraw on their own to replenish ammunition, thus losing the initative. Most retreated to Sudley Ford and were no longer a factor on the battlefield. Porter's men now found themselves in the Union front. The Marines continued to fight on ferociously into the hot afternoon as additional brigades began to fill the gaps in the lines and the enemy line moved back about a mile, giving the impression that the day was theirs.
As Confederate reinforcements arrived from Harpers Ferry by rail the Confederate resistance stiffened. Troops under Brigadier Thomas J. Jackson rallied on Henry House Hill in a stubborn defense that earned him the name "Stonewall". At 2:00pm Griffin's Battery and another battery under Captain J.B. Ricketts were ordered to occupy Henry House Hill supported by infantry and Marines. Griffin was now well to the front,his fire going harmlessly over the Confederates sheltered on the opposite side of the hill, but receiving heavy return fire from the Confederate batteries. The Marines were exposed to the same withering fire and began losing men, but they remained in support as the action grew even hotter.
As the fighting continued, an infantry regiment was observed closing on Griffin's guns. At the same time the 14th NY Infantry was moving up to protect the battery's flank, and the unknown regiment was thought to be a similar reinforcement.Griffin wanted to open fire on the unidentified unit but Major William F. Barry ordered him to hold his fire. Suddenly a murderous fire erupted against Griffin's men. The unknown unit was actually the 33rd Virginia under Colonel Arthur Cummings who disobeyed an order to hold his position and marched to within 70 yards of the Union flank wearing pre-war blue uniforms.