Complex sentence: The French and British commanders decided to make for Le Havre and the 51st (Highland) Division commander detached "Arkforce", the equivalent of two brigades, to guard the routes back to the port.
Split into simpler sentences: The French and British commanders decided to make for Le Havre. The 51st (Highland) Division commander detached "Arkforce", with the equivalent of two brigades, to guard the routes back to the port.