Complex sentence: The eastern arm of the attack would consist of I Corps with the 51st (Highland) Infantry Division, which was to cross into the Orne bridgehead and attack southwards to Cagny, south-east of Caen.
Split into simpler sentences: The eastern arm of the attack would consist of I Corps with the 51st (Highland) Infantry Division. The Highlanders would cross into the Orne bridgehead and attack southwards to Cagny, to the south-east of Caen.