Like most of the other colonies, Pennsylvania figured prominently in both the developing crisis and the war itself. Philadelphia was the site of both Continental Congresses and served as the de facto capital. British forces threatened Philadelphia in late 1776, forcing the Congress to evacuate and relocate in Baltimore, Maryland. Congress returned to Philadelphia in the spring of 1777 and remained there until Washington's defeat at Brandywine. The Congress moved to Lancaster and later to York.
In September 1777, continental forces under General "Mad" Anthony Wayne were surprised by a midnight bayonet charge by British soldiers at Paoil, Pennsylvania. This "Paoli Massacre" claimed more than 50 Americans killed and at least 100 wounded from brutal hand-to-hand fighting. The Americans bitterly resented the use of the bayonet, regarding it as a barbaric weapon. The prevailing rules of war, however, offered no such prohibition.
In December 1777, Washington forces took up winter residence at Valley Forge on the Schuylkill River near Philadelphia.
Congress resumed sessions in Philadelphia following the British evacuation in the spring of 1778.
The Pennsylvania frontier was also the scene of action. Settlers, fearing attack by combined British and Native American forces, took up a fortified position near Wilkes-Barre in July 1778. The Americans were outnumbered by more than four to one and were completely routed in what is known as the Wyoming Valley Massacre. About two-thirds of the defenders were killed in battle and many of the prisoners were tortured and killed by the Native Americans. This defeat imperiled the entire Pennsylvania frontier.