Complex sentence: With TF 18 forced to retreat, very few Allied naval forces were left in the immediate Guadalcanal area, allowing the Japanese to successfully retrieve most of their remaining ground forces, and the Allies did not realize the evacuation was happening until it was over.
Split into simpler sentences: With TF 18 forced to retreat, very few Allied naval forces were left in the immediate Guadalcanal area, allowing the Japanese to successfully retrieve most of their remaining ground forces. The Allies did not realize the evacuation was happening until it was over.