Complex sentence: At around 7:00 am, the first wave of CH-21 helicopters offloaded the first group of South Vietnamese soldiers, and they had to hold their positions until the rest of the battalions had arrived.
Split into simpler sentences: At around 7:00 am, the first wave of CH-21 helicopters offloaded the first group of South Vietnamese soldiers. They had to hold their positions until the rest of the battalions had arrived.