Complex sentence: He served admirably as quartermaster general in the Union Army during the Civil War, then devoted more time to Republican politics and quickly rose in the political machine run by New York Senator Roscoe Conkling.
Split into simpler sentences: He served admirably as quartermaster general in the Union Army during the Civil War. Arthur then devoted more of his time to Republican politics and quickly rose in the political machine run by New York Senator Roscoe Conkling.