Complex sentence: Japanese Forces were alerted to the convoy's arrival shortly before daybreak on December 28th and quickly realized that if it could be destroyed, the U.S. forces on Mindoro would be essentially cut off from their supply line.
Split into simpler sentences: Japanese forces were alerted to the convoy's arrival shortly before daybreak on December 28th. If the convoy was destroyed, the U.S. forces on Mindoro would be essentially cut off from their supply line.