Complex sentence: On July 31,1861, President Lincoln appointed him as a brigadier general in the United States Volunteers.So on September 1, he was selected by Western Department Commander, Maj. Gen. John C. Frémont, to command the District of Southeast Missouri.
Split into simpler sentences: On July 31,1861, President Lincoln appointed him as a brigadier general in the United States Volunteers. On September 1, he was selected by Western Department Commander, Maj. Gen. John C. Frémont, to command the District of Southeast Missouri.