Complex sentence: As "Dart" and "Désirée" fought at the southern end of the line, the van was under attack by the British fireships, which had been stripped of all useful materials and been converted into their original role.
Split into simpler sentences: As "Dart" and "Désirée" fought at the southern end of the line, the British fireships attacked the van. The fireships had been stripped of all useful materials and been converted into their original role.