Final answer:
The battle that occurred in 1783 when the Pennsylvania militia routed Congress was the Battle of Brandywine.
Step-by-step explanation:
The event that occurred in 1783 when the Pennsylvania militia routed Congress was the battle of Brandywine. In this battle, General Howe and his British troops defeated Washington's soldiers, took control of Philadelphia, and forced the Continental Congress to flee. The British occupied the city during the winter of 1777-1778, while Washington's army camped at Valley Forge, Pennsylvania.