Complex sentence: To patrol the Mediterranean and the Red Sea access to the Indian Ocean, Britain would work together with the French, whose own navy was the world's fourth largest, and comprised a good number of modern, powerful vessels with others nearing completion.
Split into simpler sentences: To patrol the Mediterranean and the Red Sea access to the Indian Ocean, Britain would work together with the French, whose own navy was very impressive. It was the world's fourth largest, and comprised a good number of modern, powerful vessels with many more nearing completion.