Complex sentence: Theodore Charles Benteen, an ardent secessionist, vehemently opposed his son's association with Unionists, igniting a family crisis, when Frederick joined the Union Army on September 1,1861 as a first lieutenant in the 1st Missouri Volunteer Cavalry Regiment.
Split into simpler sentences: Theodore Charles Benteen, an ardent secessionist, vehemently opposed his son's associating with Unionists. A family crisis was ignited when Frederick joined the Union Army on September 1,1861 as a first lieutenant in the 1st Missouri Volunteer Cavalry Regiment.