Thornbury Township, named after a town in Gloucestershire, England, was formed in 1687 with only five or six families, and its residents faced costly trips to Chester for legal business. As population grew in western Chester County, residents petitioned in 1766 to move the county seat inland, and fourteen years later the Pennsylvania Assembly approved a new courthouse and jail in what is now West Chester. When construction began in 1784, Chester citizens reacted so angrily that an armed confrontation occurred at the new courthouse site in the spring of 1785, though violence was narrowly avoided. The Assembly reaffirmed the removal of the courts and jail to West Chester in 1786, but continued anger in the Chester area led Pennsylvania in 1789 to create Delaware County with Chester as its seat. The dividing line ran through Thornbury and Birmingham townships, and property owners could choose which county to join, creating the irregular boundary between the two Thornbury townships; most farmers chose the new Delaware County, leaving only one-quarter of the original township's land in Chester County and making Thornbury Township, Chester County, at less than four square miles, the smallest township in Pennsylvania. In 1883, nineteen farms occupied nearly all of the township. Thornbury's greatest historical moment came in 1777, when the Battle of the Brandywine was fought on its western edge, and Squire Thomas Cheyney, born in 1732 to English-born parents who had moved there in the 1720s, gained fame that day for helping save the American army. Once almost entirely agricultural, Thornbury later came to be characterized by eighteenth- and nineteenth-century houses and post-World War II suburban subdivisions. Its population was 169 in 1800, 298 in 1950, and 2,678 in 2000.