September 1944
| 10 | September | Finland signed an armistice with the Soviet Union. |
| 11-16 | September | Second Quebec Conference (code name OCTAGON) [Roosevelt and Churchill]. |
| 12 | September | -Romania surrendered. -German troops evacuated Mytilene (Aegean). |
| 17 | September | Operation MARKET-GARDEN began (Allied airborne landings at Arnheim). |
Mediterranean
On 4 September, an air raid was carried out on Genoa by 168 B17 bombers dropping 1965x500lb bombs. Aradam, Sparide, UI 16 (former Murena), UI 20 (former Grongo) were sunk, and the large transport submarines UI 1 (former R.10) and UI 2 (former R.11) were damaged. Wolframio was sunk during an air raid on La Spezia.
On 6 September, the German Navy decided to stop work on Italian transport submarines and cancel the project of building type XXIII U-boats in the Mediterranean.
On the 19th of the month, U-407 was the last U-boat to be sunk in action in the Mediterranean. Five days later, the last two U-boats in the Mediterranean, U-565 and U-596, were scuttled at Skaramanga.
Atlantic
On 11 September, Da Procida (T.V. Francesco Castracane degli Antelminelli), escorted by USS PCE-846, sailed from Gibraltar for Bermuda to join SUBRON SEVEN for anti-submarine training.
The same day, the escort carrier Croatan (CVE-25) and her escorts (Task Group 22.5) departed from Bermuda, where they had been exercising, to escape an approaching hurricane. The submarines Dandolo, Onice, and Tito Speri remained behind and weathered the storm. The destroyers Warrington (DD-383) and Dupont (DD-152) were less fortunate. The former, a veteran of the Pacific war, sank with heavy loss of life some 200 miles east of Cape Hatteras. Dupont was badly damaged but reached port.
On 15 September, Marea (T.V. Attilio M. Russo), with USS S-82 and escorted by F-101 and PCE-869, sailed from New London for Port Everglades (Florida) and later to Key West to conduct anti-submarine training there.
Giada (T.V. Mario Barazzuoli), who had concluded her anti-submarine exercises in Colombo, departed for home on 11 September. She was escorted by the sloop Eritrea, and they both reached Taranto on 23 October.