Complex sentence: On 24 December, the right and centre attacks bogged down but on the left a platoon advanced to the sea but when it found itself isolated, out of contact and under fire from its own guns, it was forced to withdraw.
Split into simpler sentences: On 24 December, the right and centre attacks bogged down. However, on the left, a platoon advanced to the sea but when it found itself isolated, out of contact and under fire from its own guns, it was forced to withdraw.