British Leave Philadelphia
As a consequence of the French involvement in the war, the British government decided to abandon Philadelphia and concentrate their main army at New York. The British felt that the post in NY was less susceptible to French naval attack and American offenses. The British army was also directed to suspend offensive operations pending the outcome of negotiations between the American Congress and the British Carlisle Commission. The intent of the Commission was to offer a peaceful settlement to the war as granted by the British government. In short, this accord offered the Americans all of their principal demands except independence.
During the interim period (February to May 1778) the British army remained very idle in Philadelphia. When Gen Washington heard of the impending British evacuation he stepped up patrols in the area. This aggressive posture greatly denied the British access to the much needed forage and provisions they pursued in the Philadelphia country side. As the British came closer to being in a state of siege, they struck back vigorously using light troops and partisan corps such as Major John Graves Simcoe's Queen's Rangers.
On 18 May, 1778, Gen Washington detached Lafayette and 2,100 men from his main army and sent them to take up a position at Barren Hill near Philadelphia. Their mission was to ensure the denial of British foraging parties from molesting the country side. The posture of Lafayette's position left him very exposed to British attack, as realized by the British. The British mounted an offensive early on 19 May with the intent of wiping out Lafayette's detachment and thereby one-third of Washington's army.
However, Lafayette's scouts detected the presence of the offensive at the last moment and, after bluffing a counter-attack, Lafayette was able to successfully withdraw across the Schuylkill River. The feat of Lafayette's withdrawal was a major success in that he not only preserved his echelon from a superior force but he evaded a defeat by utilizing skillful, tactical art. It was a bolster for the Americans as the feat represented the hard, rigorous training that they had endured over the winter at Valley Forge. Conversely, the British effect was that it elevated a sense of direness to leave Philadelphia for N.Y.
The British move from Philadelphia toward NY, via New Jersey, is the action that set up the fateful encounter between the Americans and the British. The American main army aggressively pursued the British retreat and made their eminent contact in Monmouth, New Jersey.