She was built in Belfast, Ireland in 1905 and was the first of the Royal Mail Steam Packet Company's fleet of "A-liners"[7] that worked regular routes between Southampton and South American ports including Buenos Aires.

Owen Philipps became chairman of RMSP in 1903 and quickly addressed the company's need for larger ships on its South America route.

[4] Apart from being larger again, they differed from Aragon and her first four sisters by having three screws instead of two, and by making limited use of the turbine propulsion that Phillips and Parsons had discussed a few years earlier.

[10] From the 1850s RMSP passenger liners had served a regular route between Britain and the River Plate ports in South America.

[16] 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.

[17] Towards the end of 1912 the Admiralty decided to match the German policy by arming some British passenger liners, starting with RMS Aragon.

[18] 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.

[19] 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.

"[19] On 25 April 1913 Aragon left Southampton as Britain's first defensively armed merchant ship (DAMS), carrying two QF 4.7-inch (120 mm) naval guns on her stern.

Aragon's sister ship RMS Amazon was made the next DAMS, and in the following months further RMSP "A-liners" were armed.

[23] On 11 April she left Alexandria for the Aegean island of Lemnos, where French and British ships were assembling in the large natural harbour of Moudros in final preparation for the landings.

[22][23] On 13 April 1915 Aragon's troops transferred to the cargo steamer SS River Clyde[4] in preparation for the landing at Cape Helles 10 days later.

On 13 February Aragon left Moudros for Malta, taking troops on leave including four officers and 270 men of the 63rd (Royal Naval) Division (RND).

In December 1916 she sailed from Kilindini Harbour in the British East Africa Protectorate, reaching Durban on Christmas Day.

[4] She took about 2,200 troops[1] to reinforce the Egyptian Expeditionary Force in the Palestine Campaign against the Ottoman Empire, plus about 150 military officers, 160 VADs and about 2,500 bags of Christmas mail.

[4] On approach to the port Attack zig-zagged ahead to search the channel for mines while Aragon waited in Alexandria Roads.

[21] The armed trawler HMT Points Castle approached Aragon flying the international flag signal "Follow me".

"[21]Aragon and Attack were in Alexandria Roads[30] about 8 miles (13 km)[4] or 10 miles (16 km) outside the port, awaiting permission to enter, when at about 1100 hrs[4] the German Type UC II submarine SM UC-34 torpedoed Aragon,[1][3] hitting her port side aft[1] and causing extensive damage in her almost empty number 4 hold.

Aragon released her life rafts[4] but the explosion had smashed one of her lifeboats[31] and her increasing list prevented her crew from launching some of the remainder.

[31] By now there was an increasing number of men in the water, and trooper James Werner Magnusson of the New Zealand Mounted Rifles saw an injured soldier struggling in the very rough sea.

First, Lance-Sergeant Canfor (himself injured by the explosion) called the roll, then men were detailed to cut away the life rafts while the rest sang.

[33] We felt that all our friends were drowning before our eyes.About 15 minutes[4] after the torpedo struck Aragon, her Master, Captain Bateman, gave the order from her bridge "Every man for himself".

[29] One soldier, Sergeant Harold Riddlesworth of the Cheshire Regiment, repeatedly dived from the destroyer into the sea to rescue more survivors.

[29] Aragon's surviving lifeboats now ferried hundreds of survivors to the trawlers, where the VADs "worked unceasingly and with great heroism" to tend the many wounded.

One was Ernest Horlock, a Royal Field Artillery Battery Sergeant Major who had received the VC for "conspicuous gallantry" shown on the Western Front in 1914.

[39] A month later he told the Master of an Australian troopship, the converted AUSNC liner HMAT Indarra, that as Aragon sank Captain Bateman shouted from her bridge to Attack's commander that he would demand an enquiry into his ship having been ordered out of port.