Complex sentence: The turning point was the battle centred around the slow Convoy ONS 5 (April--May 1943), when 43 merchantmen escorted by 16 warships were attacked by a pack of 30 U-boats.
Split into simpler sentences: The turning point was the battle centred around slow convoy ONS 5 (April--May 1943). Made up of 43 merchantmen escorted by 16 warships, it was attacked by a pack of 30 U-boats.