Her husband was involved tangentially in the conquest of California, the annexation of which occurred as a result of the Mexican-American War. He served as the 3rd Military Governor, in 1847. At the time of the court-martial of Frémont, during which he attempted to defend his actions in the Bear Flag Revolt, Jessie gave birth to a son, Benton Frémont, on July 24, 1848, in Washington, D.C. The baby's death, within the year in St. Louis, she blamed on her husband's accuser, General Kearny.