Defensively equipped merchant ship

Defensively equipped merchant ship (DEMS) was an Admiralty Trade Division program 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.

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. For example, the East Indiamen class of ships were constructed from the ground up with defence in mind, with their heavy armament making them equivalent to naval ships of the line. Once the threat passed after the end of the Napoleonic Wars in 1815, armed merchant ships like East Indiamen were replaced with faster and lighter unarmed ships such as clippers.

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, which 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.

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