The Chin hills formed a natural defensive barrier into Burma, which Slim would have preferred to outflank by an amphibious operation by landing further down the coast of Burma, but demands of the war in Europe meant the necessary landing craft were not available, forcing Slim to devise plans for advancing into Burma the hard way through the Chin hills. At the same time, the Japanese 15th Army, which formed the main striking force of the Burma Area Army, had grown from four divisions at the beginning of 1943 to eight divisions by the end of 1943 as the Japanese made preparations for invading India, which increased the difficulties of an overland advance into Burma. By 1943, the Burma Death Railway, which cost the lives of thousands of slaves who built it, was finished; this allowed the Japanese to reinforce the Burma Area Army, and made invading India barely possible.