Complex sentence: The British Mark I was a tracked vehicle developed by the British Army during World War I and the world's first combat tank, entering service in August 1916, and first used in action on the morning of 15 September 1916.
Split into simpler sentences: The British Mark I was a tracked vehicle developed by the British Army during World War I and the world's first combat tank. The Mark 1 entered service in August 1916, and first used in action on the morning of 15 September 1916.