[6] Shortly after Aquitania entered service, the First World War broke out, during which she was first converted into an auxiliary cruiser before being used as a troop transport and a hospital ship, notably as part of the Dardanelles Campaign.

The White Star Line's Olympic, Titanic and the upcoming Britannic were larger than the latest Cunard ships, Mauretania and Lusitania, by 15,000 gross register tons.

Cunard needed another liner for its weekly transatlantic express service, and elected to copy the White Star Line's Olympic-class model with a slower but larger and more luxurious ship.

[11] With Aquitania's keel being laid at the end of 1910, the experienced Peskett took a voyage on Olympic in 1911 so as to experience the feel of a ship reaching nearly 50,000 tonnes as well as to copy pointers for his company's new vessel.

[4] Eighty lifeboats, including two motorised launches with Marconi wireless equipment, were carried in both swan-neck and newer Welin type davits.

On 14 May, she reached Mersey and stayed at a port there for fifteen days, during which she underwent a final major cleaning and finishing in preparation for her maiden voyage.

[10] Although Aquitania lacked the lean, yacht-like appearance of running mates Mauretania and Lusitania, the greater length and wider beam allowed for grander and more spacious public rooms.

[11] In the years prior to the First World War, Mewès was charged with the decoration of HAPAG's trio of giant new ships, Imperator, Vaterland, and Bismarck, while Davis was awarded the contract for Aquitania.

Indeed, Davis must be given credit for the Carolean smoking room and the Palladian lounge; a faithful interpretation of the style of architect John Webb.

[27] Fifteen days earlier, the German liner SS Vaterland, being the largest ship in the world at the time, was put into service.

Average speed for the voyage, a distance of 3,181 nautical miles (5,891 km; 3,661 mi) measured from Liverpool to the Ambrose Channel lightship, was 23.1 knots (42.8 km/h; 26.6 mph),[16] taking into account a five-hour stop due to fog and the proximity of icebergs.

[31] Upon arrival at her home port, she underwent minor modifications, which took into account observations made during the two first crossings (this was typical for a liner after its first round trip).

[34][4] After being idle for a time, in the spring of 1915 she was recalled by the Admiralty and converted into a troopship, and made voyages to the Dardanelles, sometimes running alongside Britannic or Mauretania.

[2][36] In 1918, now under the command of James Charles, the ship was back on the high seas in troopship service, conveying North American troops to Britain.

Many of these departures were from the port of Halifax, Nova Scotia where the ship's dazzle paint scheme was captured by artists and photographers, including Antonio Jacobsen.

[52] Author C. R. Bonsor writing in 1963 states, "from 1936 on it became necessary to squeeze the maximum speed out of Aquitania in order to make her a suitable running mate for Queen Mary thus 24 knot passages became regular".

[55] On 10 April 1935, Aquitania went hard aground on Thorne Knoll in the Solent near Southampton, England, but with the aid of ten tugboats, on the next high tide the ship was freed.

[2] When the new liner RMS Queen Elizabeth was due to enter service in 1940, newspapers speculated that Aquitania would be scrapped that year, since the QE replaced her on the North Atlantic route.

[57] These ships, often termed "Monsters" until London requested the term be dropped, were Aquitania, Queen Mary, Queen Elizabeth, Mauretania (II), Île de France and Nieuw Amsterdam with "lesser monsters" being other large ex-liners capable of independent sailing with large troop capacity that accounted for much of the troop capacity and deployment, particularly in the early days of the war.

[2] Meanwhile, a massive transport of Australian and New Zealand troops to Suez and North Africa, with possible diversion to the United Kingdom if events required, was in planning with the numbered convoys to be designated as "US" with the large Atlantic liners assigned a role.

[65] Now repainted battleship grey, in November 1941 Aquitania was in the British colony of Singapore, from which she sailed to take part indirectly in the loss of the Australian cruiser HMAS Sydney.

[67] On 28 December Aquitania and two smaller transports departed Sydney with 4,150 Australian troops and 10,000 tons of equipment for Port Moresby, New Guinea.

Because her deep draft was hazardous in Australian and intermediate ports in the Pacific Islands,[70] she spent March and April 1942 transporting troops from the west coast of the U.S. to Hawaii.

[75] As part of the major redeployment of Australian troops from North Africa to the defence of Australia and start of offensive operations in the Southwest Pacific.

[76] By the buildup for the invasion of Europe in 1944 troop deployments to Britain depended heavily on Aquitania and the other "Monsters" and no allowance could be made for interruption of their service for other transport requirements.

The unit was sent by "devious routes" by train to Jersey City where under cover of darkness they boarded a ferry crossing to the covered pier 86 in New York where a band played and the Red Cross served their last coffee and doughnuts as they boarded "N.Y. 40", the New York Port of Embarkation code designation for Aquitania, which got underway the morning of 29 January 1944 with some 1,000 Navy and 7,000 Army personnel for arrival at Gourock, Scotland 5 February.

[78] In eight years of military work, Aquitania sailed more than 500,000 miles, and carried nearly 400,000 soldiers,[2][79] to and from places as far afield as New Zealand, Australia, the South Pacific, Greece and the Indian Ocean.

A long-standing rumor was made that during a corporate luncheon held on the ship, a piano had fallen through the roof of one of the dining rooms from the deck above.

Aquitania carried 1.2 million passengers through an active sea career that spanned nearly 36 years, making her the longest-serving Express Liner of the 20th century.

In 35 years of service Aquitania had sailed more than 3 million miles and apart from one or two early Allan Line steamers no other ship served for as long in a single ownership.