The Naval Intelligence Division had solved a message intercepted from U-boat U-111 that was returning to France, concerning a rendezvous with U-67 and U-68 in the Bay of Tarrafal on the island of Santo Antão.

[1]: 725  They had little success; one factor in this was that the Allies had penetrated the German Enigma code system, and were able to garner up-to-date intelligence (Ultra) on the whereabouts of marauding U-boats.

[3] Cape Verde was neutral Portuguese territory and Tarrafal Bay was a remote region of it; BdU reasoned it would be an eminently suitable site for the meeting.

The arrangement for the RV were transmitted encrypted by radio; the messages were received and decoded at Bletchley Park, including an indiscrete reference by the captain of U-111 to Tarrafal Bay as the rendezvous site.

After transferring torpedoes, and making social exchanges, both Merten and Kleinschmidt chose to stand out to sea, in order to gain sea-room, while awaiting U-67.

Catching sight of U-68, Ingram quickly set up a torpedo attack, but before he could fire, his lookouts spotted U-111 heading towards him on a collision course.

Judging it was too close for a torpedo attack, Ingram determined to ram her, while Müller-Stöckheim, deciding against a crash-dive, backed engines to avoid the oncoming submarine.

U-67 was unable to continue; her torpedo tubes were inoperable and she was ordered to return, still carrying the sick crew-member whose plight had sparked the incident.

[1]: 385 The arrival of Clyde in Tarrafal bay on the same evening that U-boats were due to rendezvous was immediately seen as too unlikely to be a coincidence; BdU ordered a full investigation of security, including a check to see whether Enigma had been breached.

The action had jeopardized Britain's fragile advantage in the intelligence war, and was saved only by the German navy's complacency concerning its Enigma code.