Boundary disputes were quite common between the colonies, but few got as violent as the boundary line war between Pennsylvania and Maryland from 1730-1738. Pennsylvania’s charter (1681) provided for its southern boundary as follows: “on the South by a Circle drawne at twelve miles’ distance from New Castle Northward and Westward unto the beginning of the fortieth degree of Northern Latitude, and then by a streight Line Westward”. Subsequent surveys established that Dover was a full twenty-five miles south of the 40th Parallel. Maryland insisted on the 40th Parallel which would have made Philadelphia a Maryland town. Pennsylvania pushed for a boundary at 39 degrees, 36 minutes which would have taken a strip out of what is northern Maryland. The dispute simmered for decades breaking out into open conflict in the 1730s with the settlement of the Conejohela Valley west of the Susquehanna River. Maryland and Pennsylvania settlers in the disputed territory quickly came into conflict with raids and counter raids by the militias of the two colonies. The leader of the Maryland settlers was Thomas Cresap, a tough and fearless man as the French would later have reason to attest during the French and Indian War. Cresap was captured by the Pennsylvanians. Upon being paraded through the streets of Philadelphia prior to being imprisoned, Cresap remarked: “Damn it, this is one of the prettiest towns in Maryland!”.
Only the personal intervention of King George II stopped the fighting with a treaty between the two colonies being signed in London on May 25, 1738, providing for an exchange of prisoners and a provisional boundary between the colonies being drawn 15 miles south of Philadelphia. The permanent boundary would not be established until the drawing of the Mason-Dixon line by surveyors Charles Mason and Jeremiah Dixon.