Complex sentence: The American Civil War broke out four days after Parker took office in 1861 and he enlisted in a pro-Union home guard unit, the 61st Missouri Emergency Regiment, he had reached the rank of corporal by the end of the war.
Split into simpler sentences: The American Civil War broke out four days after Parker took office and he enlisted in a pro-Union home guard unit, the 61st Missouri Emergency Regiment. He had reached the rank of corporal by the end of the war.