General George Washington and the Continental Army spent the winter of 1777-8 at Valley Forge. As the winter wore on the supply situation was brought under control and something approaching a proper issue of equipment and rations was made to the troops. Memorably the Prussian officer General Steuben trained the American regiments in a form of European battle drill, devised and adapted to suit American troops.
The British army spent the winter in Philadelphia. Lt. Gen. Howe returned to England, relieved of his appointment in command in America at his own request, to be replaced by General Clinton. Clinton arrived with orders to evacuate Philadelphia and concentrate the British forces at New York.
On 18th June 1778 the British army with artillery, supplies and the Loyalist populace of the city left Philadelphia and began the 100 mile march to New York. Their marching column stretched over 12 miles. Washington decided that he was going to harass and inflict some damages to the British troops. He was also confident that newly trained Continental Army will teach British a lesson.
On June 26, Washington ordered Lee to get ahead of the British column and bring the British withdrawal to a halt until he could bring up the main strength of the American army along the Monmouth Road. On June 28, General Washington, bringing the main American army along the Monmouth road, encountered, not the rear of the British column, but Lee’s regiments retreating in considerable disorder with the British advancing behind them. Memorably this is the one occasion Washington is said to have sworn as he relieved Lee on the spot. Washington then galloped forward and began the task of rallying Lee’s disordered troops. Washington ordered General Wayne with the last of Lee’s regiments, Stewart’s 13th Pennsylvania and Ramsay’s 3rd Maryland, to form to the North of the road and hold the British advance. These regiments resisted strongly but were driven back by the British 16th Light Dragoons. Their stand gave Washington the time to bring up the rest of the American army, with artillery on Comb’s Hill ( where Molly Pitcher pitched in as gun crew ) to the South of the road enfilading the attacking British foot. Fierce fighting took place as the British attempted to drive back the American line. This was the first test of Steuben’s re-trained American Continental regiments and they repelled the British with their bayonet. As the evening wore on the British troops fell back and returned to their journey north, leaving the Americans on the field.
While the battle was a draw, the British now have a respect for the Continental Army and the Clinton's men will remain holdup in New York for the reminder of the war. The war then moves to Carolinas, where the American Army is not as well trained.
Since the exact number of engaged from both sides can't be determined exactly , many historian considered either the Battle of Monmouth (~26000 engaged ) or Battle of Brandywine (~28000 engaged ) as the largest battle of the American Revolution.