Complex sentence: Henry Flagler, a partner with John D. Rockefeller in Standard Oil arrived in St. Augustine in the 1880s and was the driving force behind turning the city into a winter resort for the wealthy northern elite.
Split into simpler sentences: Henry Flagler, a partner with John D. Rockefeller in Standard Oil, arrived in St. Augustine in the 1880s. He was the driving force behind turning the city into a winter resort for the wealthy northern elite.