Complex sentence: Assigned a support role, the 15th Battalion was held in reserve and did not take part in the attack, but the following day, it was committed to hold the gains made, relieving the New Zealanders around Gooseberry Farm.
Split into simpler sentences: Assigned a support role, the 15th Battalion was held in reserve and did not take part in the attack. The following day, it was committed to hold the gains that had been made during the attack, relieving the New Zealanders around Gooseberry Farm.