Complex sentence: When Mexican forces in California capitulated on January 13, they did not do so to Stockton or Kearny, but to Stockton's aide, Lt. Col. John C. Frémont.
Split into simpler sentences: When Mexican forces in California capitulated on January 13. However, they did not do so to Stockton or Kearny, but to Stockton's aide, Lt. Col. John C. Frémont.