Complex sentence: By 1870, the population had declined from 40,000 at the time of the United States acquisition of California, to 20,000, At that point, thousands of Indians had been murdered, raped or sold into slavery.
Split into simpler sentences: By 1870, the population had declined from 40,000 at the time of the United States acquisition of California to 20,000. Thousands of Indians had been murdered, raped or sold into slavery.