Complex sentence: From that point on the Cherokee were under intense scrutiny, and following the Killough Massacre, when at the end of 1838 President Houston was succeeded by Mirabeau B. Lamar, a hardliner in Indian matters, their fate became almost certain.
Split into simpler sentences: From that point on, and following the Killough Massacre, the Cherokee were under intense scrutiny. When at the end of 1838 President Houston was succeeded by Mirabeau B. Lamar, a hardliner in Indian matters, their fate became almost certain.