The 'Battle of Cape Bon' was a naval engagement between two Italian light cruisers and a British destroyer flotilla off Cape Bon in Tunisia (13 December 1941).

At the time of Italy’s entry into World War II in June 1940, the Italian navy was one of the largest navies in the world but was restricted to the Mediterranean Sea in all but the operations of light forces and submarines. The British possessed sufficient resources and naval might to maintain a strong presence in the area and also to replace most losses by the redeployment of ships from other theatres. This persuaded the Italian naval authorities to adopt a policy of caution and a tendency to avoid conflict. Even so, control of the Mediterranean Sea was disputed by the Regia Marina and the Royal Navy and their allies. The sea was vital for the supply from Italy of the Italian and German forces in North Africa, and also for the maintenance of Malta as a British offensive base. Without Malta, the UK could not intercept Italian convoys and thereby prevent the supply of the Axis forces in North Africa.

The possession of radar and the breaking of Italian naval codes, most especially the Boris Hagelin C38 cipher machine used by the Regia Marina, further contributed to British success. In November 1941, the supply of the Axis forces in Libya from Italy had been interrupted by the activities of Force 'K', which had destroyed several Italian convoys (most notably the 'Beta' convoy) and the loss of nearly 70% of the supplies sent to Libya, including 92% of the fuel. Force 'K', based at Malta, and ships of Admiral Sir Andrew Cunningham’s Mediterranean fleet, based at Alexandria in northern Egypt, intercepted an Axis convoy, comprising the merchant vessels Maritza and Procida escorted by two torpedo boats, sailing from Greece to Benghazi on 24 November. The convoy was about 120 miles (190 km) to the west of Crete when the merchant vessels were attacked and sunk by a British force of two cruisers and two destroyers, the torpedo boats making off once it was clear that the merchant vessels were doomed. The loss of the cargoes led the German command to report that the fuel situation of the Luftwaffe in North Africa was critical.

The Axis forces in North Africa, facing the new 'Crusader' offensive, were in urgent need of fuel and ammunitions. The Supermarina, which was the general staff of the Italian navy, at the request of the Comando Supremo, which was the supreme command of the Italian armed forces, therefore developed an emergency plan to shift supplies using warships. Alberico da Barbiano and Alberto di Giussano, light cruisers of the 4a Divisione Incrociatori under the command of Ammiraglio di divisione Antonino Toscano, being fast but too lightly protected for employment with the battle fleet, were selected for this task.

Alberico da Barbiano (Toscano’s flagship) and Alberto di Giussano departed Taranto in southern Italy at 08.15 on 5 December 1941, reached Brindisi at 17.50 and there loaded about 50 tons of supplies, then proceeded to Palermo on 8 December, where they loaded 22 tons of aviation fuel to alleviate the shortage in Libya, which was preventing aircraft from escorting supply convoys. Carried in unsealed barrels, the fuel was placed on the stern, creating a grave risk of fire in the event the ships were hit by British gunfire and event from the discharge of the ships' own guns. The latter prevented the use of the stern turrets unless the fuel had been jettisoned. The two cruisers sailed unescorted from Palermo at 17.20 on 9 December, heading for Tripoli. At 22.56, when steaming to the north of Pantelleria island, they were spotted by a British reconnaissance aeroplane, which had been directed to the area by 'Ultra' signal intercepts and decryptions, and which now began to shadow the ships. At 23.55 Toscano, whose ships were at that time in the middle of the Sicilian Channel, decided to turn back as the surprise required for the success of the mission had vanished, much British radio traffic foreshadowed air attack, and worsening sea conditions would delay the ships, further exposing them to British attacks. Alberico da Barbiano and Alberto di Giussano reached Palermo at 08.20 on 10 December after surviving a British air attack off Marettimo. Toscano was heavily criticised by the Supermarina for his decision to abort the mission.

The 'M.41' convoy was planned for 13 December, but air cover by aircraft based in Libya would be impossible unless fuel was received from Italy, and on 12 December it was decided that the 4a Divisione Incrociatori would make another attempt to reach Tripoli. The light cruiser Giovanni delle Bande Nere was to join Alberico da Barbiano and alberto di Giussano to carry more supplies, but was prevented from sailing by a mechanical problem and her cargo was transferred to the other two cruisers. Alberico da Barbiano and Alberto di Giussano were loaded with 98 tons of aviation fuel, 246 tons of petrol, 591 tons of naphtha, 886 tons of food and 135 ratings on passage to Tripoli. The stern of Alberico da Barbiano and to a lesser extent that of Alberto di Giussano were thickly packed with so many fuel barrels that it was impossible to traverse the after gun turrets. Toscano briefed his staff and officers from both ships, and it was decided that, in case of encounter with British ships, the barrels would be thrown overboard to enable the ships to open fire. Alberico da Barbiano, Alberto di Giussano and their only escort, the torpedo boat Cigno (a second torpedo boat, Climene, had been left in the port as a result of a breakdown) departed Palermo on the northern coast of Sicily at 18.10 on 12 December. The 4a Divisione Incrociatori had been instructed to pass to the north-west of the Aegadian islands and then head for Cape Bon and follow the Tunisian coast. The ships were to steam at 22 or 23 kt to conserve fuel and deliver their cargoes at Tripoli. Air cover, air reconnaissance and defensive ambushes by motor torpedo boat were planned in order to safeguard the convoy.

Commander G. H. Stokes’s British 4^th Destroyer Flotilla, comprising the British destroyers Sikh, Maori, Legion and the Free Dutch Isaac Sweers, had departed Gibraltar on 11 December to join the Mediterranean Fleet at Alexandria. By 8 December, the British had decoded Italian C38 radio signals about the Italian supply operation and its course for Tripoli. The Royal Air Force sent a Vickers Wellington twin-engined aeroplane on a reconnaissance sortie to sight the ships as a deception, and on 12 December the 4^th Destroyer Flotilla, heading to the east from Gibraltar toward the Italian ships, was ordered to increase speed to 30 kt and effect an interception. During the afternoon of 12 December, a Cant Z.1007bis three-engined bomber of the Regia Aeronautica spotted the four destroyers heading to the east at an estimated speed of 20 kt off Algiers, and the Supermarina was immediately informed but calculated that, even in the event that the destroyers increased speed to 28 kt, they would not reach Cape Bon until around 03.00 on 13 December, about one hour after the 4a Divisione Incrociatori, so Toscano, who was informed of the sighting while still in harbour, was not ordered to increase speed or alter course to avoid them.

Following new 'Ultra' decrypts, a new reconnaissance aeroplane was sent and spotted Toscano’s ships at sunset on 12 December, after which the 4^th Destroyer Flotilla was directed to increase speed to 30 kt and intercept the two cruisers. Along with a one-hour delay that the 4a Divisione Incrociatori had accrued, and which Toscano omitted to report to the Supermarina, this speed frustrated all previous Supermarina calculations about the advantage that the 4a Divisione Incrociatori would have. At 22.23 Toscano was informed that he would possibly meet 'enemy steamers coming from Malta', and at 23.15 he ordered action stations.

The 4^th Destroyer Flotilla sighted the Italian cruisers near Cap Bon, at 02.30 on 13 December. Some 15 minutes later, 7 miles (11.25 km)) off Cape Bon, the Italian ships heard the sound of a British aeroplane (a radar-equipped Wellington, which located the ships and informed Stokes about their position), and at 03.15 they altered course to 157° to pass about 1 mile (1.6 km) off Cape Bon. Five minutes later, Toscano suddenly ordered full speed and an alteration to 337°, effectively reversing course. This sudden change disrupted the Italian formation, as neither Cigno, which was about 2 miles (3,2 km) ahead of the cruisers, nor Alberto di Giussano, which was following Alberico da Barbiano received the order, and while Alberto di Giussano saw the flagship reverse course and imitated her, but remained misaligned, Cigno did not notice the course change until 3.25, when she also reversed course, but remained considerably to the rear of the two cruisers.

Stokes’s destroyers were just off Cape Bon by that time and had spotted the Italian ships. Arriving from astern, under the cover of darkness and using radar, the British ships steamed close inshore and surprised the Italians, who were further out to sea, by launching torpedoes from short range. The course reversal accelerated the approach between the two groups and the Allied destroyers attacked together. Sikh fired her guns and four torpedoes against Alberico da Barbiano at a range of less than 1,100 yards (1005 m) Legion did the same, Isaac Sweers opened fire on Alberto di Giussano, and Maori fired six torpedoes against Alberto di Giussano. Toscano ordered an increase to full speed and fire to be opened. Alberico da Barbiano also started a turn to port on the order of Capitano di vascello Giorgio Rodocanacchi, but at 03.22, before any her guns other than some machine guns were able to open fire, was hit by a torpedo below her foremost 6-in (152.4-mm) twin turret, which caused the cruiser to list to port. Alberico da Barbiano was then raked with machine gun fire, which killed or wounded many men and set fire to the fuel barrels, and was also struck by a second torpedo in the engine room.

At 03.26 Maori fired two torpedoes at Alberico da Barbiano and opened fire with her 4.7-in (119.4-mm) guns, hitting the Italian ship’s bridge. The cruiser was hit soon after by another torpedo, possibly from Legion, in the stern, and Alberto di Giussano was also hit by a torpedo and gunfire, being left disabled. The loom of the land behind the Allied destroyers made it impossible for the Italians to see their opponents, and Alberto di Giussano managed to fire only three salvoes. In five minutes both cruisers had been disabled. Alberico da Barbiano rapidly listed to port, while fires quickly spread all over the ship and into the sea from the floating fuel; the crew abandoned ship. At 03.35, Alberico da Barbiano capsized and sank in flames, with Toscano, Rodocanacchi and another 532 men still on board. Alberto di Giussano was left dead in the water with fires raging; the crew struggled to keep the ship afloat, but she too had to be abandoned, breaking in two and sinking at 04.20 with the loss of 283 men. After a brief encounter with Isaac Sweers, Cigno rescued nearly 500 survivors; others reached the coast and another 145 men were later saved by Italian motoscafo armato silurante motor torpedo boats. The Italian losses amounted to 817 men.

Toscano’s decision to reverse course has never been fully explained, but various possibilities have been suggested. He may have decided to turn back after realising that he had been spotted from the air, as he did on 9 December; however, a course toward the Aegadian islands would have made more sense than the north-westerly course he in fact ordered. The course change was ordered more than 30 minutes after the cruisers had been spotted, and Toscano may have wanted to mislead the air reconnaissance about his real course, wait for the aeroplane to leave and then turn again for Tripoli. He may have thought, from the aircraft noise, that torpedo bombers were on their way, and wished to get into waters farther away from the shore and from Italian minefields in order to gain freedom of manoeuvre. Toscano ordered his gunners to stand by; he may have known that Allied destroyers were astern of his ships, and he wanted to avoid presenting his stern to them because his after turrets were obstructed by the fuel barrels.