It was accepted that German commerce raiders were too well armed for converted ocean liners equipped with obsolete guns but nothing else was available to the Royal Navy until later in the war.

Voltaire patrolled the Mediterranean enforcing contraband regulations then sailed to Halifax, Nova Scotia to escort convoys across the Atlantic.

On 16 July, the British ship Wendover fought back and managed to transmit a report before being sunk by gunfire and torpedoes, after the crew was taken prisoner, bringing the total on board to over 200 men.

In an engagement that lasted for about four hours, Thor inflicted serious damage and received two hits in return; Alcantara making slowly for Brazil.

[7] On 26 September, in the south Atlantic, the Norwegian ship Kosmos III was sunk, then the British Natia on 8 October; 'Thor now carrying more than 360 prisoners.

Several British cruisers were concentrated off the River Plate and Rio de Janeiro but Thor met few ships, apart from German supply vessels, for three months.

[9] In early April 1941, the AMC HMS Voltaire sailed from Trinidad in the Caribbean for Freetown, Sierra Leone and to search west of the Cape Verde islands for commerce raiders en route.

[12] On 4 April Thor was back in the area to the north-east of Brazil, posing as a Greek freighter and at 06:15, her lookouts reported smoke on the horizon to port.

The ship was HMS Voltaire (Captain James Blackburn); Kähler turned toward the vessel, not realising that it was an AMC, and the British assumed that they had met a small freighter and closed on her to check.

Kähler was surprised when the ship returned fire and replied with two guns; this time, an AMC was well within range and converging when the engagement began.

Blackburn abandoned the bridge when it was ablaze and went to the stern to command one of the two remaining 6-in guns but only managed to hit the top of Thor's mast.

[13] When the Germans announced the sinking of Voltaire, the Admiralty sent the Canadian AMC Prince David to search, which found wreckage half-way between Trinidad and the Cape Verde Islands.