Owen Philipps had become Chairman of RMSP in 1903, and over the next five years had introduced five new, larger ships on the company's premier route between Southampton and the east coast of South America.

Two years later, on 27 June 1914, she gave a short publicity cruise for UK dignitaries, South American ambassadors and a former prime minister of China.

On 16 August 1914 she was of the coast of Brazil with more than 1,000 people aboard when the German 14,349 GRT auxiliary cruiser Kaiser Wilhelm der Grosse intercepted her[7] with the order "Stop or I will open fire".

[8][9] In April 1915 the Admiralty requisitioned Arlanza and her "A-series" sisters Avon, Andes and Alcantara for conversion into armed merchant cruisers.

Andes and Alcantara were also commissioned into the 10th Cruiser Squadron, which joined the Northern Patrol[11] that was part of the First World War Allied naval blockade of the Central Powers.

[10][13] Arlanza's bow dipped 10 to 12 feet (3.0 to 3.7 m) lower in the water, but the bulkhead doors between her watertight compartments were closed and prevented her from sinking.

An accident when lowering one of the lifeboats dropped a number of Russian representatives and Royal Navy sailors into the icy water, but all were rescued.

[14] A party from the ship travelled by reindeer sledge across 10 miles (16 km) of snowy tundra to the Sami village of Yukanski for supplies.

[16] A reduced crew of nine officers and 100 men stayed aboard Arlanza in the White Sea, making temporary repairs to enable her to return to Ireland.

On 3 June 1916 the Thames tug Racia reached Kola Inlet to tow the cruiser back to Belfast.

[11][18] This included a trip to South America in 1918, carrying the members of a special diplomatic mission from the British Foreign Office.

[11] She remained continuously in regular liner service, whereas each of her surviving sisters now undertook cruising at least part-time and in some cases full-time.

In 1924 she took Harland and Wolff Chairman Lord Pirrie to the River Plate to assess facilities for the newly ordered RMSP liners Asturias and Alcantara.

the Italian Government, Lloyd's of London and RMSP awarded medals and made presentations to a number of Arlanza's crew for the bravery and outstanding seamanship they showed in the rescue.

In Montevideo HMS Exeter's Royal Marines Band played her out of port and the event was broadcast by radio throughout Uruguay.