Complex sentence: He sees Highway as an anachronism in the "new" Marine Corps, and assigns him to shape up the reconnaissance platoon, which is made up of undisciplined Marines who had been allowed to slack off by their previous platoon sergeant.
Split into simpler sentences: He sees Highway as an anachronism in the "new" Marine Corps, and assigns him to shape up the reconnaissance platoon. "Recon" is made up of undisciplined Marines who had been allowed to slack off by their previous platoon sergeant.