The Royal Navy of the Unite Kingdom is the oldest of the British armed services (and is therefore known as the Senior Service). From the early 18th century to the middle of the 20th century, 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. With the collapse of the Soviet Union, its role for the 21st century has returned to focus on global expeditionary operations.