Complex sentence: Before the opening of the Conference, the British had already occupied Constantinople, the French had marched into Cilicia, and the Italians landed in Antalya on the southern coast as well as being promised parts on the western coasts including İzmir.
Split into simpler sentences: Before the opening of the Conference, the British had already occupied Constantinople and the French had marched into Cilicia. Moreover, to the Italians had been promised Antalya on the southern coast as well as parts on the western coasts including İzmir.