returned to Cagliari on August 15, 1937. During the second one, Diaspro departed Naples on August 25 to patrol off Cape Oropesa. On September 1, 1937 Diaspro sighted near Benicarló a tanker heading to Alicante. Not being able to catch up with the ship under water, Diaspro attacked the tanker on the surface by launching two torpedoes. The ship crew spotted them, managed to maneuver and avoid them altogether, and made an attempt to ram the submarine. Diaspro fired two more torpedoes which hit the ship in the middle and sank it. The captain of the submarine believed this was a 6987 GRT Republican tanker heading to Alicante under the presumably false name , as the crew appeared to be Romanian. In reality the ship was recently acquired by a British-based operator, and the tanker was carrying petroleum from Constanta to Valencia. Diaspro returned to Naples on September 5, 1937. At the time of Italy's entrance into World War II Diaspro was assigned to the 72nd Squadron (VII Submarine group) based at Cagliari. Her first mission in Tyrrhenian Sea was uneventful. In July 1940 she first patrolled northwest of Asinara and then off the island of La Galite. In August 1940 she patrolled off Cap Bougaroûn. On September 1, during British Operation "Hats" Diaspro was on patrol in an area between Cape Spartivento and the island of La Galite. She managed to close to within ten miles from the British Force H naval group but could not attack because it was detected and repeatedly attacked by the aircraft of British aircraft carrier and forcing the submarine to dive. On November 9, 1940, she left Cagliari to patrol off the island of La Galite along with four other submarines with the task of intercepting a British convoy, part of Operation "Coat" but failed to sight any enemy ships. On November 14, she was deployed along with submarines and as a screen to British force part of Operation "White". Located about 315 miles from the island of La Galite, she was only able to detect a minor ship from the British force, but failed to attack it. On January 21, 1941 Diaspro and patrolled off the northern Tunisian coast. Diaspro was on three missions in February, May and June 1941, east of the island of La Galite, about twenty miles north of Cape Blanc, and south of Sardinia, respectively, but without any