Returning from Lancaster on January 17, 1765, Charles Mason, one of two English astronomers running the new border between Pennsylvania and Maryland, fell in with a former county sheriff named Samuel Smith. Smith told him what he had seen nearly thirty years before: a Maryland settler named Thomas Cresap, defending a house on the Susquehanna River with fourteen men, besieged by about fifty-five more who set the house on fire when the defenders would not surrender. One man died coming out. Mason wrote down that night what Smith had told him: the people near "the supposed Boundary Line" had been at open war.
The line Cresap had been willing to die over had never actually been run. Maryland's 1632 charter put its northern edge at the fortieth parallel. Pennsylvania's charter, signed by Charles II in 1681, described its own southern edge as running along that same parallel until it met a circle of twelve miles' radius around New Castle, then following the circle's arc to Delaware Bay. Nobody in London had checked whether the two descriptions actually agreed with each other: the fortieth parallel passes about thirteen miles north of that circle, so the boundary the king had granted never closed. An agreement signed in 1732 broke down in disputes over method; once border violence like the Cresap siege spread, King George II in 1738 ordered a temporary line that stood as the boundary for twenty-five years while the case crawled through an English Chancery court that took fifteen years to decide. When the court finally ordered a resurvey in 1750, the colonies' surveyors still could not agree on method: Maryland wanted distances chained up hills and down into valleys, Pennsylvania wanted them measured level, and the same ground could yield two different borders.
By 1763 the two families gave up on local surveyors and asked the Astronomer Royal to recommend someone who could check a disputed line against the sky instead of leaving it to chain and argument alone. He named Mason, a longtime assistant at the Greenwich observatory, and Jeremiah Dixon, with whom Mason had already observed a transit of Venus from the Cape of Good Hope for the Royal Society. The two men still ran the boundary by chain, in short arcs corrected for the earth's curve, but controlled its position with a zenith sector that tracked stars almost overhead: more than once over four years, a star reading showed the line had drifted south of true, and the surveyors measured back north by that distance before pushing on. A stone was set at every mile for the first 132 miles west, until hauling cut stone grew impractical; beyond that, wooden posts and earth mounds carried the line the rest of the way.
By October 1767, Mason and Dixon had carried the line 233 miles west, past the ground Cresap had defended, until their native guides refused to go further; the last 31 miles to Pennsylvania's southwest corner waited until 1784, for other surveyors to finish. A boundary decided in an English courtroom was only a claim, enforced by whichever side put more armed men on the disputed ground. A boundary checked against the stars was something any trained surveyor, anywhere, could check by the same method and reach the same answer. Cresap had been willing to burn inside his own house over a line that, in 1736, no one had yet gone out and measured.