Complex sentence: Fremont's men were called the Osos, the local insurgents of the day, and his men had jailed the Sonoma alcalde and put the town under siege in the Bear Flag Revolt.
Split into simpler sentences: Fremont's men were called the Osos, the local insurgents of the day. The Osos jailed the Sonoma alcalde and put the town under siege in the Bear Flag Revolt.