Complex sentence: On 5 December 1941, when the threat of Japanese invasion became more likely, the plan was modified to use the forces available, it was to be put into action as soon as an attack was imminent.
Split into simpler sentences: On 5 December 1941, when the threat of Japanese invasion became more likely, the plan was modified to use the forces available. It was to be put into action as soon as an attack was imminent.