Complex sentence: On July 15th, the Regulators have gathered in Lincoln, organized by McSween, and receive word that a large posse is on its way into town, so instead of attempting to flee they elect to remain in town to meet the posse.
Split into simpler sentences: On July 15th, the Regulators gathered in Lincoln, organized by McSween, and received word that a large posse was on its way into town. Instead of attempting to flee they elected to remain in town to meet the posse.