The Royal Navy of the United Kingdom is the oldest of the British armed services (and is therefore known as the Senior Service). From the 18th century to the middle of the 20th century (250 years), it was the largest and most powerful navy in the world, playing a key part in establishing the British Empire as the dominant power of the 19th and early 20th centuries. In World War II, the Royal Navy operated almost 900 ships. During the Cold War, it was transformed into a primarily anti-submarine force, hunting for Soviet submarines, mostly active in the GIUK gap. It suffered massive losses during the Twilight War, and has only a few servicable surface vessels and no submarines.