Complex sentence: Theoretically this gave Britain a 4.5:1 manpower advantage, by comparison the Union's manpower advantage over the Confederacy in the American Civil War was only 2.5:1.
Split into simpler sentences: Theoretically this gave Britain a 4.5:1 manpower advantage. By comparison the Union's manpower advantage over the Confederacy in the American Civil War was only 2.5:1.