Complex sentence: The Arawe landing was scheduled for 15 December; this was the earliest date by which the air bases needed to support the landing that could be made operational and also gave the landing force time to conduct essential training and rehearsals.
Split into simpler sentences: The Arawe landing was scheduled for 15 December as this was the earliest date by which the air bases needed to support the landing could be made operational. This time frame also gave the landing force time to conduct essential training and rehearsals.