In Southern Tunisia. Also known as the Battle of the Horseshoe. To quote the plaque: "History records this as one of the greatest and most terrible battles fought by the Regiment in the Second World War." But Wikipedia has no knowledge of it, skipping from 6-10 March (Operation Capri) to 19 March (Operation Pugilist) with no mention of the Battle of Mareth on 16/17 March. Described on the monument and full accounts are given at WW2Talk and Grenadier Guards.