The Royal Navy (RN) is the United Kingdom's naval warfare force. Although warships were used by the English kings from the early medieval period, the first major maritime engagements were fought in the Hundred Years War against the Kingdom of France. The modern Royal Navy traces its origins to the early 16th century; the oldest of the UK's armed services, it is known as the Senior Service.
From the middle decades of the 17th century, and through the 18th century, the Royal Navy vied with the Dutch Navy and later with the French Navy for maritime supremacy. From the mid 18th century, it was the world's most powerful navy until surpassed by the United States Navy during the Second World War. The Royal Navy played a key part in establishing the British Empire as the unmatched world power during the 19th and first part of the 20th centuries. Due to this historical prominence, it is common, even among non-Britons, to refer to it as «the Royal Navy» without qualification.
Following World War I, the Royal Navy was significantly reduced in size, although at the onset of World War II it was still the world's largest. By the end of the war, however, the United States Navy had emerged as the world's largest. During the Cold War, the Royal Navy transformed into a primarily anti-submarine force, hunting for Soviet submarines and mostly active in the GIUK gap. Following the collapse of the Soviet Union, its focus has returned to expeditionary operations around the world and remains one of the world's foremost blue-water navies. However, twenty-first century reductions in naval spending have led to a personnel shortage and a reduction in the number of warships.
The Royal Navy maintains a fleet of technologically sophisticated ships and submarines including an aircraft carrier, two amphibious transport docks, four ballistic missile submarines (which maintain the UK's nuclear deterrent), six nuclear fleet submarines, six guided missile destroyers, 13 frigates, 13 mine-countermeasure vessels and 22 patrol vessels. As of April 2018, there are 73 commissioned ships (including submarines) in the Royal Navy, plus 10 ships of the Royal Fleet Auxiliary (RFA); there are also five 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 vessels. It also works as a force multiplier for the Royal Navy, often doing patrols that frigates used to do. The total displacement of the Royal Navy is approximately 407,000 tonnes (603,000 tonnes including the Royal Fleet Auxiliary and Royal Marines).
The Royal Navy is part of Her Majesty's Naval Service, which also includes the Royal Marines. The professional head of the Naval Service is the First Sea Lord who is an admiral and member of the Defence Council of the United Kingdom. The Defence Council delegates management of the Naval Service to the Admiralty Board, chaired by the Secretary of State for Defence. The Royal Navy operates three bases in the United Kingdom where commissioned ships are based; Portsmouth, Clyde and Devonport, the last being the largest operational naval base in Western Europe.