On June 26, 1778, Washington chose to send 4,000 men as an advance force to strike at the British rear guard as they departed Monmouth Courthouse, in order to delay the British withdrawal until the main American force could give battle.
Charles Lee, as Washington's senior subordinate, was initially appointed commander of the advance force, but turned it down because of his doubts about the plan. However, when the force was increased to 5,000 and the command offered to the Marquis de Lafayette, Lee changed his mind and insisted on the command.
Lee met with his subordinates but failed to give them proper orders, resulting in a piecemeal and disorganized attack on June 28 against the British rear guard under General Charles Cornwallis. After several hours of fighting in the hot weather, several American brigades executed a tactical retreat, which developed into a general withdrawal. The British rear guard counterattacked and Lee ordered a retreat, which rapidly became a rout.
Washington, advancing with the main force along the Monmouth road, encountered Lee's fleeing troops and finally Lee himself, with the British in hot pursuit. After a heated exchange with Lee, Washington relieved him of command and sent him to the rear. He then rallied Lee's troops, who delayed the British pursuit until the main force could take up positions further to the west.
The remnants of Lee's forces then withdrew to the main American force, where the Continental Army troops were positioned behind the West Ravine on the Monmouth Courthouse - Freehold Meeting House Road. Washington drew up his army with Greene's division on the right, Major General Stirling's division on the left, and most of Lee's former force, now under Lafayette, in reserve. In front of his lines, Wayne commanded various elements of Lee's force. Artillery was placed on both wings, with the right wing in position to enfilade the advancing British.
The British came on and attacked Stirling's left wing with their light infantry and the 42nd (Black Watch) Regiment in the van. They were met by a storm of fire from Stirling's Continentals. The battle raged back and forth for an hour until three American regiments were sent though woods to enfilade the attacking British right flank. The attack was successful and sent the British back to reform.
Foiled on the left, Cornwallis personally led a heavy attack against Greene's right wing, with a force comprising British and Hessian grenadiers, light infantry, the Coldstream Guards and another Guards battalion, and the 37th and 44th Regiments. The attack was met by enfilading fire from Thomas-Antoine de Mauduit du Plessis's four 6-pound cannons on Combs Hill, as well as accurate volleys from Greene's Continental regiments. The British persisted up the ravine slope but within minutes five high-ranking officers and many men were down from heavy fire. The attackers recoiled down the slope.
During Cornwallis' abortive attack on Greene, another British force made up of grenadiers, light infantry and light dragoons hit Wayne's forward force, who were protected behind a long hedge. Three times the British were driven back by Wayne's grapeshot and bullets: but an overwhelming fourth attack overlapped Wayne's position and forced his units to fall back to the main American line.
The British made no further attempts on the main American line, although cannonading from both sides continued until 6 p.m. At this point, the British fell back to a strong position east of the Ravine. Washington wanted to take the offensive to the British and attack both flanks, but darkness brought an end to the battle.
The British rested and then resumed their march to the northeast during the night. Washington wanted to resume the battle the next day but in the morning found that the British had withdrawn during the night, continuing their march without incident to Sandy Hook and arriving there on June 30. The British force was then transported by the Royal Navy across Lower New York Bay to Manhattan.
The battle was a tactical British victory, as the rearguard successfully covered the British withdrawal. The battle effectively ended in a draw, as the Americans held their ground, but the British were able to get the army and supplies safely to New York.
The British official casualty return reported 65 killed, 59 dead of 'fatigue', 170 wounded and 64 missing. The American official return stated 69 killed, 161 wounded and 132 missing (37 of whom were found to have died of heat-stroke). Other estimates increase the losses to 1,134 British and 500 American casualties.
After the battle the British continued their march eastwards until they reached Sandy Hook. From there they were taken by boat to New York City where they began preparing the city's defenses in expectation of an attack. D'Estaing's fleet arrived just too late, narrowly missing a chance to trap Clinton's army at Sandy Hook. Plans to attack New York were abandoned, and it remained the principal base for British forces until 1783. Instead D'Estaing sailed north to participate in a Franco-American assault on the British garrison at Newport, Rhode Island, which ended in failure. Monmouth was the last major battle in the northern theater, and the largest one-day battle of the war when measured in terms of participants. Lee was later court-martialed at the Village Inn located in the center of Englishtown, where he was found guilty and relieved of command for one year. The verdict was approved by the Continental Congress by a close vote. Many months later, Lee wrote a strongly worded letter to Congress in protest but Congress closed the affair by informing him that it had no more need of his services. Lee never held another military command and died in 1782.