Complex sentence: The cost was a massacre of both French and Spaniards: of the 307 men in the rearguard only 74 survived, caked with blood; uniforms in tatters; bayonets chipped and blunted.
Split into simpler sentences: The cost was a massacre of both French and Spaniards. Of the 307 men in the rearguard only 74 survived, caked with blood; uniforms in tatters; bayonets chipped and blunted.