Founded February 22, 1838, from portions of surrounding counties, Mercer County has a historical impact that reaches back to the pivotal battles of the American Revolutionary War. On the night of December 25–26, 1776, General George Washington led American forces across the Delaware River to attack the Hessian forces in the Battle of Trenton on the morning of December 26, also known as the First Battle of Trenton. Following the battle, Washington crossed back to Pennsylvania. He crossed a third time in a surprise attack on the forces of General Charles Cornwallis at the Battle of the Assunpink Creek, on January 2, 1777, also known as the Second Battle of Trenton, and at the Battle of Princeton on January 3. The successful attacks built morale among the pro-independence colonists.