War at sea in the 20th century is defined by the decline in the absolute dominance of seapower in global strategy and geopolitics and by rapid technological change in naval propulsion, communications and sensors, and weapons.

In the 18th and 19th centuries, Britain had emerged as the most powerful maritime and imperial power, enjoying marked advantages over her continental rivals. She possessed superior internal communications, was a major tariff-free market, and did not have to maintain a large standing army: being protected by the 'wooden walls ' of the Royal Navy. Her expanding global maritime communications facilitated the movement of men, material, goods, and information more easily than the internal land communications of the European Great Powers; and the inward flow of capital stimulated by her expanding mercantile trade, was an important factor in the phenomenon of the 18th century British industrial revolution. Europe, moreover, is ringed with water from the White Sea to the Black Sea, enabling the Royal Navy to implement close blockades, mount amphibious assaults, logistically support armies, or exert diplomatic pressure against hostile powers by the flexibility of seapower.

Ultimately Great Britain defeated France, her main opponent, by a combination of seapower and military power applied on a global scale in theatres of operation in Europe, North Africa, India and North America: and superior external maritime communications and industrial and financial muscle. For nearly a century afterwards, British global pre-eminence, the so-called 'Pax Britannica', was founded on her battlefleets and merchant navy, her industrial and financial supremacy, her world empire and numerous naval bases, and the geo-strategic dominance of naval power.
Yet by the beginning of the 20th century these strategic parameters were challenged by new patterns in world power, and by technological developments. New centres of industrial and naval power - the United States, Germany and Japan - had emerged as Great Powers, to join the established Powers of France, Russia, Italy and Austria-Hungary. By 1914 Imperial Germany possessed the second largest navy in the world. Based, as it was, across the North Sea, it posed a direct threat to metropolitan Britain and the heart of British global power.

At the same time, the geo-strategic dominance of seapower was eroded by the new 19th century technologies of the railway, telegraph, combustion engine and radio. These enhanced the internal communications and strategic viability of the Continental Powers, particularly the United States and Russia. Evolving 20th developments of the aeroplane, submarine, mechanized armies and logistics, electronics, and missile technology further consolidated the global influence of these vast self-sufficient centres of industrial and military power which were invulnerable to traditional naval strategies of Nelsonic battles of annihilation, close blockade and bombardment.

New technologies in propulsion, communications and sensors, and weapons also influenced the waging of war at sea. Diesel-engined submarines, an outstanding new weapon system, appeared during the first decade of the 20th century, whilst coal-fired engines were increasingly replaced by oil-fired turbines in surface warships. The latter were smaller and more powerful, and needed less manpower: 40% less weight in oil was needed to give the same range as coal. The overall performance of warships was increased, and armament, armour, and internal protection were improved. Refuelling was facilitated and refuelling at sea multiplied the effectiveness of naval forces in campaigns fought in extensive theatres of operation, such as the Atlantic (1939-45) and Pacific (1941-45) campaigns.

The first nuclear-powered fleet submarines (SSN) and ballistic missile submarines (SSBN) were built in the 1950s followed by major surface warships. The endurance of these vessels was limited only by crew fatigue and the amount of stores and ordnance carried. Small gas turbine-engined warships appeared in the 1950s, and now power most major warships giving excellent performance and a fast reaction capability.
Equally revolutionary were developments in communications and electronics. These dramatically enhanced the command, control and communications, and search-and-destroy capabilities of all warships. The first Marconi sets were installed in Royal Navy warships in 1899 and by 1906 every capital ship in the service was equipped with a wireless. By 1914 nearly all warships in the Royal Navy were so equipped. Admiralty command and control of its fleets was exercised through transmitting stations as widely dispersed as Cleethorpes, Horsea Island, Gibraltar, Malta, Hong Kong and the Falklands Islands.

During the First World War (1914-18) wireless communications and its corollary, signals intelligence (Sigint), the latter epitomised in the Admiralty's Room 40, became a major factor in the war at sea. By the 1920s more efficient valve radios were increasingly replacing spark transmitters, and research on Asdic (Sonar), the ability to detect submarines by electronically transmitted sound underwater continued and in the 1930s work on radar intensified. During the Second World War major warships were comprehensively equipped with search, navigational and gun laying radars and associated IFF units (Identification Friend or Foe); LF, MF, HF and VHF radios; and Asdic if engaged on anti-submarine duties. Again, Sigint centred on Ultra and Bletchley Park was of major importance. Post-war, the electronics fit of warships has been continually upgraded: including computer systems and advanced radars and sonars to combat air, submarine and missile threats.

During the First World War the battleship and the big gun were the maritime arbiters, although threatened by the submarine, an outstanding weapon system, the torpedo and the mine. By the time of the Second World War (1939-45) aircraft, both land- and carrier-based, were of great importance and proved as deadly as the submarine in the war at sea. During 1941 the aircraft carrier emerged as the most powerful surface ship, relegating the battleship to a secondary role. Radar and radio equipped aircraft, shore or carrier based, were powerful weapons in the war at sea, and aircraft carriers could exert a sea control influence over a radius of 250 miles. Post-war the air-, ship-, or submarine-launched missile has ultimately become the most powerful destructive weapon. The dominant warship is now the SSN and SSBN operating in a total stealth environment and the submarine-launched nuclear tipped ballistic missile is the most terrifying of all maritime weapons. Yet secondary gun systems are fitted in surface warships and carrier strike aircraft, and the cheap mass-produced sea-mine, air- or ship-launched, is a very formidable weapon.