Defensively equipped merchant ship (DEMS) was an Admiralty Trade Division programme established in June 1939, to arm 5,500 British merchant ships with an adequate defence against enemy submarines and aircraft. The acronym DEMS was used to describe the ships carrying the guns, the guns aboard the ships, the military personnel manning the guns, and the shore establishment supporting the system. This followed a similar World War I program of defensively armed merchant ships (DAMS).

The program was distinct from armed merchant cruiser program, which were warships converted from civilian vessels, operated by the Royal Navy itself.

In the eighteenth and nineteenth centuries, European countries such as Spain, France, the Netherlands and Britain armed their merchant ships to prevent capture by pirates, enemy commerce raiders and privateers when they conducted overseas trade. The most heavily armed were ships carrying valuable cargo back from the Far East. Notably the East Indiamen class of ships were constructed from the keel up with defence in mind, with their heavy armament making some of the most powerful examples equivalent to naval Fourth-rate ships of the line. After the end of the Napoleonic Wars in 1815, these were replaced for some of the balance of the 19th century with faster and lighter unarmed ships such as clippers that, in theory, could outrun any threat when blockade running or carrying smaller quantities of the most valuable cargoes long distance.

From the turn of the 20th century, growing tensions between Europe's Great Powers included an Anglo-German naval arms race that threatened the security of merchant shipping. In December 1911 a memo from Winston Churchill, recently appointed as First Lord of the Admiralty, proposed that the utility or otherwise of arming British merchant ships "for their own defence" be ascertained. The Admiralty created a Committee on the Arming of British Merchant Vessels under Captain Alexander Duff, that reported in May 1912. In October 1912 Admiral Sir Francis Bridgeman became Churchill's First Sea Lord, and that October Bridgeman warned the Committee of Imperial Defence that "the Germans were arming their merchant ships, nominally for the protection of their own trade, but more probably in order to attack ours." The ships being armed by the Kaiserliche Marine were passenger liners that were fast enough to serve as auxiliary cruisers, and they would indeed be used as raiders in WWI, though there were not as many as the British expected.

The British Admiralty intended to have armed merchant cruisers of its own through the potential wartime conversion of vessels such as the RMS Lusitania and RMS Mauretania to outright warships. A second plan was to experiment with having civilian ships armed for their own protection, starting with the Royal Mail Steam Packet Company passenger liner RMS Aragon. She was due to carry naval guns from December 1912, but within the British Government and Admiralty there was uncertainty as to how foreign countries and ports would react. Many merchant ships had been armed in the 18th century and it had never been made illegal, but Britain feared that foreign authorities might refuse to let armed British merchant ships enter port, or might intern them. In January 1913 Rear Admiral Henry Campbell recommended that the Admiralty should send a merchant ship to sea with naval guns, but without ammunition, to test foreign governments' reaction. A meeting chaired by Sir Francis Hopwood, Civil Lord of the Admiralty agreed to put guns without ammunition on a number of merchant ships "and see what happens." Sir Eyre Crowe was at the meeting and recorded "If nothing happens, it may be possible and easy, after a time, to place ammunition on board." To emphasize the defensive nature of the guns, they were to be mounted aft, so that they could be used only while making an escape.

In March the policy was made public, and in April it was implemented. On 25 April 1913 Aragon left Southampton carrying two QF 4.7-inch (120 mm) naval guns on her stern. The Admiralty planned to arm Houlder Brothers' La Correntina similarly if the reaction were favourable. Governments, newspapers and the public in South American countries that Aragon visited took little notice and expressed no concern.

There was more criticism in Britain, where Commander Barry Domvile, Secretary to the Committee of Imperial Defence, warned that the policy undermined Britain's objection to the arming of German merchant ships. Domvile predicted that arming merchant ships would be ineffective, and would lead only to a second maritime arms race alongside the naval one. Gerard Noel, a former Admiral of the Fleet, told Churchill that were a merchant ship ever to fire its guns it could be accused of piracy. Churchill replied by drawing a distinction between merchant ships armed as auxiliary cruisers and those armed only for self-defence.

Privately Churchill was more concerned, and in June 1913 he directed Admiralty staff to "do everything in our power to reconcile this new departure with the principles of international law". However, the policy continued. Aragon's sister ship RMS Amazon was made the next DAMS, and in the following months further RMSP "A-liners" were armed. They included the newly built Alcantara, that in the First World War did indeed serve as an armed merchant cruiser.