Complex sentence: To facilitate the defence, the important islands on the western edge of the atoll would eventually be linked by a light railway across causeways built up between the islands, but this was not operational until much later in the war.
Split into simpler sentences: To facilitate the defence, the important islands on the western edge of the atoll would be linked by a light railway across causeways built up between the islands. This was not operational until much later in the war.