Complex sentence: The constabulary-type Army of the antebellum frontier days was based on a high level of mobility, while the Union Army that won the U.S. Civil War massed combat power in a series of campaigns aimed at the heart of Confederate strength.
Split into simpler sentences: The constabulary-type Army of the frontier days was based on a very high level of mobility. However, the Army that won the U.S. Civil War was based on mass. The Union Army successfully massed combat power where it was needed in a series of campaigns aimed at the heart of Confederate strength.