In 1940 she was renamed Uckermark[2] and used as supply tanker for the battleships Scharnhorst and Gneisenau during Operation Berlin before sailing to Japan in September 1942 as a blockade breaker.

[3] Altmark (Captain Heinrich Dau) was assigned to support Admiral Graf Spee during her raid in the South Atlantic between September and December 1939.

After Admiral Graf Spee was heavily damaged by British cruisers in Battle of the River Plate and subsequently scuttled by her crew, in the Río de la Plata in December 1939, Altmark attempted to return to Germany, steaming around the north of Great Britain and then within the Norwegian littoral.

Hitler had long feared Norway would be insufficiently resolute to protect the German iron-ore traffic that passed legitimately along the Norwegian littoral and after earlier discussions with Admiral Erich Raeder and Vidkun Quisling had decided already on 14 December 1939 to ultimately invade the country.

[5]: 244 The British justification for the attack on the Altmark was set out in a Note to the Norwegian Government from Foreign Secretary Lord Halifax dated 10 March 1940.

The question remains unresolved to this day as to whether, as the Hague Conventions stood in 1940, a warship could legitimately seek immunity from attack in neutral waters by widely varying its course to reach them.

During Operation Berlin, which involved the battleships Scharnhorst and Gneisenau between January and March 1941, Uckermark, under Captain Zatorski, was a supply ship and scout attached to the squadron.

On 9 September 1942 she left France for Japan with a cargo of vegetable oil and fuel, supplying the auxiliary cruiser Michel on the way, arriving at Yokohama on 24 November 1942.