Complex sentence: At the time that Layforce was raised, the British were in the ascendency in the theatre, having largely defeated the Italians, and it was felt that the Commandos could be employed in the capture of the island of Rhodes.
Split into simpler sentences: At the time that Layforce was raised, the British were in the ascendency in the theatre, as they had largely defeated the Italians. It was felt that the Commandos could be employed in the capture of the island of Rhodes.