The Royal Navy (RN) is the principal naval warfare service branch of the British Armed Forces. Tracing its origins to the 16th century, it is the oldest service branch and is known as the Senior Service. From the end of the 17th century until well into the 20th century it was the most powerful navy in the world, playing a key part in establishing the British Empire as the dominant world power. Due to this historical prominence, it is common – even among non-Britons – to refer to it as "The Royal Navy" without qualification.
Following victory in the First World War the Royal Navy was significantly reduced in size, although at the onset of the Second World War it was still the largest in the world. By the end of the Second World War the United States Navy had emerged as the world's largest. During the course of the Cold War, the Royal Navy transformed into a primarily anti-submarine force, hunting for Soviet submarines, mostly active in the GIUK gap. Following the collapse of the Soviet Union its focus has returned to global expeditionary operations around the world.
The navy maintains a fleet of technologically sophisticated ships including an aircraft carrier (though currently without fixed-wing aircraft until c.2020), an amphibious assault ship, two amphibious transport docks, four ballistic missile submarines (which maintains the UK's nuclear deterrent), seven nuclear fleet submarines, five guided missile destroyers, 13 frigates, 15 mine-countermeasure vessels and 24 patrol vessels. As of March 2013, there were 78 commissioned ships in the Royal Navy, plus 13 commissioned ships of the Royal Fleet Auxiliary (RFA). There are also 6 Merchant Navy ships available to the RFA under a private finance initiative. The RFA replenishes Royal Navy warships at sea, and augments the Royal Navy's amphibious warfare capabilities through its three Bay-class landing ship dock vessels. The total displacement of the Royal Navy is approximately 340,000 tonnes (775,000 tonnes including the Royal Fleet Auxiliary and Royal Marines).