Complex sentence: In 1789, the city became the county seat for the newly created Delaware County (whereupon Chester County became landlocked, with West Chester as its county seat), but the county seat was moved to the borough of Media in 1851.
Split into simpler sentences: In 1789, the city became the county seat for the newly created Delaware County (whereupon Chester County became landlocked, with West Chester as its county seat). The county seat was moved to the borough of Media in 1851.