Complex sentence: The 53rd--whose performance during the fighting had come to be seen in less than favourable light--was reduced to being used for reinforcements and work parties, however, the 39th subsequently returned to the battle when the forward troops were under pressure.
Split into simpler sentences: The 53rd--whose performance during the fighting had come to be seen in less than favourable light--was reduced to being used for reinforcements and work parties. The 39th returned to the battle when the forward troops were under pressure.