The 'Campaign for the Adriatic Sea' was a relatively small naval campaign fought between the Greek, Yugoslav and Italian navies, the German navy, and the Mediterranean naval forces of the UK, France and the Yugoslav partisan movement (7 April 1939/15 May 1945).

On 7 April 1939, on the orders of Benito Mussolini, the Italian dictator, Italian forces occupied Albania and overthrew King Zog, and the country was then annexed to the Italian empire. Naval operations in the Adriatic Sea took the form primarily of transport to Albania from the ports of southern Italy, as well as coastal bombardment in support of the landings on the Albanian coast. The Italian naval forces involved in the invasion of Albania included the battleships Giulio Cesare and Conte di Cavour, three heavy cruisers, three light cruisers, nine destroyers, 14 torpedo boats, one minelayer, 10 auxiliary ships and nine transport vessels. The ships were divided into four groups, which carried out landings at Vlorë, Durrës, Sarandë and Shëngjin.

When Italy entered World War II, on 10 June 1940, the Italian navy’s main naval bases in the Adriatic Sea were Venice, Brindisi and Pola. The northern Adriatic was under the control of the Northern Adriatic Autonomous Naval Command, headquartered in Venice and headed by Ammiraglio di Divisione Ferdinando di Savoia, Duca di Genova, who was succeeded by Ammiraglio di Divisione Emilio Brenta shortly before the Italian armistice of September 1943 with the Allies. The southern Adriatic was under the control of the Southern Adriatic Naval Command, headquartered in Brindisi and headed by Ammiraglio di Squadra Luigi Spalice. Ammiraglio di Divisione Vittorio Tur was the naval commander of Albania, with headquarters in Durrës. Naval commands also existed in Pola, which was the home of the Italian navy’s submarine school, and on the island of Lussino (Losinj).

On the outbreak of war, the Italian naval forces in the Adriatic Sea included the destroyers Carlo Mirabello and Augusto Riboty and the 7a Squadriglia Cacciatorpediniere (Angelo Bassini, Nicola Fabrizi, Enrico Cosenz and Giacomo Medici) in Brindisi, the 15a Squadriglia Cacciatorpediniere (Palestro, Confienza, San Martino and Solferino) in Venice, the gunboat Ernesto Giovannini in Pola, the 2a Squadriglia MAS (four motor torpedo boats) in Pola, the 3a Squadriglia MAS (two boats) in Brindisi, and several minelayers as the relatively shallow waters of the Adriatic Sea were particularly favourable for mine warfare. Seven submarines were based in Brindisi: Balilla, Domenico Millelire, Enrico Toti and Antonio Sciesa of the 40a Squadriglia Sommergibile), Brin of the 42a Squadriglia Sommergibile, Anfitrite of the 44a Squadriglia Sommergibile and Ondina of the 48a Squadriglia Sommergibile.

The following Greco-Italian War lasted from 28 October 1940 to 30 April 1941 and was part of World War II. Italian forces invaded Greece from Albania and initially made limited gains. At the outbreak of hostilities, the Greek navy comprised the old cruiser Georgios Averof, 10 destroyers (four old 'Thiria' class, four relatively modern 'Dardo' class and two new 'Greyhound' class), several torpedo boats and six old submarines. Faced with the formidable Regia Marina, its role was limited primarily to patrol and convoy escort duties in the Aegean Sea. This was essential both for the completion of the Greek army’s mobilisation and the overall resupply of the country as the convoy routes were threatened by Italian aircraft and submarines operating from the Dodecanese islands group. Nevertheless, the Greek ships also carried out limited offensive operations against Italian shipping in the Strait of Otranto.

On the Italian side, convoy operations between Italy and Albania were under the responsibility of the Comando Superiore Traffico Albania (Maritrafalba, or high command for traffic with Albania), established in Valona on 5 October 1940 and initially commanded by Capitano di Vascello Romolo Polacchini. The Maritrafalba's forces included two elderly 'Mirabello' class destroyers, 11 equally old torpedo boats of the 'Palestro', 'Curtatone', 'Sirtori', 'Generali' and 'La Masa' classes, four more modern 'Spica' class torpedo boats of the 12a Squadriglia Cacciatorpediniere, four auxiliary cruisers and four motor torpedo boats of the 13a Squadriglia MAS. The main Italian supply routes were from Brindisi to Valona and from Bari to Durrës.

Greek destroyers carried out three bold but fruitless nocturnal raids on 14/15 November 1940, 15/16 December 1940 and 4/5 January 1941. The main Greek successes came from the submarine force, which managed to sink some Italian transport vessels, although the Greeks also lost a submarine in the process, but the Greek submarine force was too small to be able to made a serious impact on the supply lines between Italy and Albania: between 28 October 1940 and 30 April 1941 Italian ships made 3,305 passages across the Strait of Otranto carrying 487,089 military personnel (including 22 field divisions) and 584,392 tons of supplies while losing only seven merchant vessels and one escort ship. Although the Regia Marina suffered severe losses in capital ships to the British during the 'Judgement' torpedo-bomber raid on Taranto, Italian cruisers and destroyers continued to operate covering the convoys between Italy and Albania. Also, on 28 November, an Italian squadron bombarded Corfu, while on 18 December and 4 March, Italian naval units shelled Greek coastal positions in Albania.

The only surface engagement between the Regia Marina and the Royal Navy took place on the night of 11/12 November 1940, when a British squadron of three light cruisers and two destroyers attacked an Italian return convoy consisting of four merchant vessels escorted by the auxiliary cruiser Ramb III and the torpedo boat Nicola Fabrizi, in the 'Battle of the Strait of Otranto'. All four merchant vessels were sunk, and Nicola Fabrizi was severely damaged. In March 1941, Fairey Swordfish single-engined torpedo bombers of the Fleet Air Arm, based at Paramythiá in north-western Greece, raided the harbour of Valona on several occasions, sinking one merchant vessel, one torpedo boat and the hospital ship Po. The Regia Aeronautica then discovered the airfield used by the British warplanes and bombed it, knocking it out for some weeks. In April the airfield became operational once again and another raid on Valona was carried out, sinking two additional merchant vessels. On the same day, however, German forces launched their 'Marita' invasion of Greece, and the base at Paramythiá was bombed by the Luftwaffe and permanently destroyed.

The German 'Unternehmen 25' invasion of Yugoslavia began on 6 April 1941 and ended with Yugoslavia’s unconditional surrender on 17 April. Germany, Italy and Hungary then occupied and dismembered the Yugoslavia. When Germany and Italy attacked Yugoslavia, the Yugoslav navy had a strength of three destroyers, 10 torpedo boats and two submarines as the most effective units of its fleet. One other destroyer, Ljubljana, was in dry dock at the time of the invasion, and she and her anti-aircraft guns were used in defence of the fleet base at Kotor. The remainder of the fleet was useful only for coastal defence and local escort and patrol work.

Kotor (Cattaro) was close to the Albanian border and the Italo-Greek front there, but Zara (Zadar), an Italian enclave, was to the north-west of the Yugoslav coast, and to prevent a beach-head being established there, the destroyer Beograd, four of the old torpedo boats and six motor torpedo boats were despatched to Sibenik, 50 miles (80 km) to the south of Zara, in preparation for an attack. The attack was to be co-ordinated with that of the 12^th 'Jadranska' Division and two odreds (combined regiments) of the Yugoslav army attacking from the Benkovac area, supported by the air attacks of the 81^st Bomber Group. The Yugoslav forces launched their attack on 9 April, but by 13 April Italian forces under Generale d’Armata Vittorio Ambrosio had counterattacked, and were in Benkovac by 14 April.The naval element of the attack faltered when Beograd was damaged by near misses from Italian aircraft off Sibenik. With her starboard engine out of action, she limped to Kotor, escorted by the remainder of the force, for repair. Italian air raids on Kotor badly damaged the minelayer Kobac, which was beached to prevent her from sinking.

The maritime patrol floatplanes of the Yugoslav air force flew reconnaissance and attack missions during the campaign, as well as providing air cover for minelaying operations off Zara. Their operations included attacks on the Albanian port of Durrës, as well as attacks on Italian supply convoys to Albania. On 9 April, one Dornier Do 22K single-engined floatplane notably took on an Italian convoy of 12 vessels with an escort of eight destroyers crossing the Adriatic Sea during the day, attacking in the face of intense anti-aircraft fire. No Italian ships were sunk by Yugoslav forces, however, but an Italian tanker was claimed damaged by a near miss off the Italian coast near Bari. Most of the small Yugoslav fleet (the old cruiser Dalmacija, three destroyers, six torpedo boats, three submarines, 11 minelayers and several auxiliary vessels) was seized by Italian ground forces in the bases at Split and Kotor, and later recommissioned under the Italian flag. Only four Yugoslav ships escaped capture: the submarine Nebojsa and two motor torpedo boats sailed to Allied-controlled ports, while Zagreb was scuttled to prevent her capture.

After the end of 'Marita', Italy controlled the entire eastern coast of the Adriatic Sea through the annexation of much of Dalmatia, the Italian occupation zone of the puppet state of Croatia, the Italian governorate of Montenegro, and the Italian puppet régime in Albania.

The naval forces of the Yugoslav partisan movement were formed as early as 19 September 1942, when partisans in Dalmatia established their first naval unit. Initially comprising adapted fishing boats, this gradually evolved (especially after the armistice between Italy and the Allies) into a force able to conduct complex amphibious operations. This event is considered to be the foundation of the Yugoslav navy. At its peak during World War II, the Yugoslav partisans' navy comprised nine or 10 armed ships, 30 patrol boats, almost 200 support ships, six coastal batteries, and several partisan detachments on the islands, around 3,000 men.

After the Italian capitulation of 8 September 1943, following the Allied invasion of Italy, the partisans retook most of the coast and all of the islands. On 26 October, the Yugoslav partisan navy was organised first into four and later six Pomorsko Obalni Sektor (maritime coastal sectors). The task of the naval forces was to secure supremacy at sea, organise the defence of the coast and the islands, and attack Axis sea traffic and forces on the islands and along the coasts.

After the fall of Greece and Yugoslavia, the complete Italian control of both coasts of the Adriatic combined with the distance from British naval and air bases dictated a temporary end to all British air and surface operations in the Adriatic Sea. From the spring of 1941 to September 1943, Royal Navy activity in the Adriatic was thus limited to submarine operations, mainly in the southern part of this sea, but Italian convoys across the Adriatic Sea suffered only negligible losses. Between June 1940 and September 1943, only 0.6% of the personnel and 0.3% of the supplies shipped from Italy to Albania and Greece were lost: two-thirds of these losses were caused by submarines, mostly British. The Royal Navy lost four submarines in the Adriatic Sea, most likely to mines. British surface ships re-entered the Adriatic after the September 1943 armistice, when the much weaker forces of the German navy were their only opponents.

As an initial undertaking after the Italian to the Allied camp, in 'Wolkenbruch' (i) the Germans rushed to occupy the ports of the northern part of the Adriatic Sea: these ports were Trieste, Fiume (Rijeka) and Pola (Pula), and established the Operationszone Adriatisches Küstenland (operational zone Adriatic coast), headquartered in Trieste, on 10 September. This comprised the provinces of Udine, Gorizia, Trieste, Pula, Fiume and Lubiana (Ljubljana). Since an Allied landing in the area was anticipated, the Operationszone Adriatisches Küstenland (Operational Zone Adriatic Coast also hosted a substantial German military element, the Befehlshaber Operationszone Adriatisches Küstenland under the command of General Ludwig Kübler. On 28 September 1944, these units were redesignated as the XCVII Corps. German marine units were also formed.

Vizeadmiral Joachim Lietzmann was the Kommandierender Admiral Adria (commanding admiral Adriatic). The area of operation initially ranged from Fiume to Valona, and the area of the western coast was under the jurisdiction of the Deutsches Marinekommando Italien (German naval headquarters Italy). The demarcation line between the two naval commands corresponded with the line between Heeresgruppe 'F' in the Balkans) and Heeresgruppe 'C' in Italy, and also provided the border between the so-called Italian Social Republic (revived Fascist state in northern Italy) and the so-called Independent State of Croatia. Soon, at Lietzmann’s insistence, the area of operation was extended to include the whole of Istria to the mouth of the Tagliamento river, corresponding with the boundary line between the Italian Social Republic and the area of the Operationszone Adriatisches Küstenland.

One of the first operations was 'Herbstgewitter' (i), which took the form of German troop landings on the islands of Krk, Cres and Losinj in November 1943. The Germans used some old ships, such as the cruiser Niobe and the auxiliary cruiser Ramb III. During the action, the islands were cleared of partisan forces and Niobe, in company with two Schnellboote, managed to capture a British military mission on the island of Losinj.

Gradually the German naval forces in the Adriatic Sea were enlarged, mostly with former Italian ships found in an advanced stage of construction in the yards at Fiume and Trieste. The strongest naval unit was the 11^th Sicherungsflottille: established during May 1943 in Trieste as the 11^th Küstenschutzflottille, in December 1943 it was designated as the [pe]11^th Sicherungsflottille, and was employed in the protection of maritime communications in the Adriatic, mostly against partisan naval attacks. On 1 March 1944, the flotilla was extended and redesignated as the 11^th Sicherungsdivision.

After Italy’s capitulation until the end of 1943, German forces advanced into Dalmatia, and from this time the Allies undertook a major evacuation of civilian population fleeing the German occupation of Dalmatia, and in 1944 moved the refugees to the El Shatt camp in Egypt.

By 1944, only the island of Vis (Lissa in Italian) remained unoccupied by the Germans, and Yugoslav and British troops were tasked with preparing its defences against the later-cancelled German 'Freischütz' (ii) operation. The island is about 14 miles (23 km) long and 8 miles (13 km) wide, with a mainly hilly terrain, and a plain in the centre covered with vines, part of which had been removed to make way for an airstrip about 750 yards (690 m) long, from which four Supermarine Spitfire single-engined fighter-bombers of the Balkan Air Force were operating. At the island’s western end, is the port of Komiza, while at the other end is the port of Vis: these were connected by the only good road running across the plain. Vis was organised as a stronghold, and held until the end of World War II.

In 1944 Tito’s headquarters moved to the island, and eventually more than 1,000 British troops were included in the defence of Vis. The British forces on the island were called the Land Forces Adriatic, and were under the command of Brigadier George Daly. The force consisted of Nos 40 and 43 (Royal Marine) Commandos of the 2^nd Special Service Brigade, the 2/Highland Light Infantry and support troops. Operating from the two ports were several Royal Navy craft. Tito’s forces numbered about 2,000. Vis served as the political and military centre of the liberated territories until the liberation of Belgrade late in 1944.

A remarkable figure was a Canadian officer, Captain Thomas G. Fuller, who in 1944 took command of the 61^st Motor Gun Boat Flotilla. Operating from Vis, the flotilla supplied the partisans by pirating German supply ships. Fuller’s vessels sank or captured 13 German supply boats, was involved in 105 fire fights and another 30 operations in which there was no gunfire. Characteristically for the Yugoslav operations theatre, Fuller attributed a good part of his success to the blood-curdling threats uttered by the Yugoslav partisan who manned the motor gun boat’s loud hailer: a 400-ton schooner was captured with its entire cargo after its crew had surrendered without a struggle because of the explanation of what would be done to them personally, with knives, if they disobeyed.

British naval forces in the Middle East operating in the Adriatic Sea were under the command of the Flag Officer Taranto and Adriatic and Liaison with the Italians. All the naval forces were controlled from Taranto and operated in close co-ordination with the coastal attack operations undertaken by the Balkan Air Force. The Yugoslavs used the British vessels for the transport of materials and men, and most especially to make landings on the islands off the Dalmatian coast to liberate them from German occupation.

During the Vis period, partisans carried out several seaborne landings on Dalmatian islands with help of Royal Navy and commandos: these islands included Korčula, Solta ('Detained'), Hvar ('Endowment'), Mljet ('Farrier') and Brač ('Flounced').

The French navy was also involved during the first half of 1944, with their 10^th Light Cruiser Division (the three 'Fantasque' class large destroyers Le Fantasque, Le Terrible and Le Malin) making high-speed sweeps in the Adriatic Sea and destroying German convoys. One notable action was the 'Battle off Ist' on 29 February 1944, when a German convoy force of two corvettes and two torpedo boats escorted one freighter supported by three minesweepers. The French attack destroyed the freighter and one corvette in return for no loss before withdrawing.

In the second half of 1944 the Royal Navy sent a destroyer flotilla into the Adriatic. The largest engagement was that of 1 November, when two 'Hunt' class escort destroyers (Avon Vale and Wheatland) were patrolling the coastal shipping routes to the south of Lussino and sighted the German corvettes UJ-202 and UJ-208. The destroyers opened fire at a range of 4,000 yards (3660 m) and in less than 10 minutes reduced the two German ships to scrap as the pair of British ships circled the German warships and poured a devastating storm of pom-pom and small-calibre gunfire onto them. When the first corvette had been sunk. Avon Vale closed to rescue the Germans while Wheatland continued to engage the second corvette, which eventually blew up. Some 10 minutes later, the British came under fire from the German destroyer TA-20 (ex-Italian Audace), which suddenly appeared on the scene. When the two British ships directed their fire at her, the German destroyer was sunk. But as the campaign in the Adriatic Sea continued to the end of the war, the 'Hunt' class warships did not again engage larger German warships, although the German navy was constantly launching and commissioning light destroyer types from the Trieste and Fiume yards. On 14 December, Aldenham became the last British destroyer to be lost in World War II when struck a mine off the island of Skrda.

To prevent an Allied maritime advance into the northern part of the Adriatic Sea in the last two years of World War II, Germany laid thousands of mines and blocked all ports and canals. Many minefields were situated in the open sea. Minesweeping was executed by British ships equipped with special minesweeping technology, and on 5 May 1945 the 'Shakespeare' class trawler Coriolanus hit a mine while sweeping off Novigrad.

The Allies, initially under a French initiative by Général d’Armée Maxime Weygand, planned landings in the Thessaloníki area of northern Greece. Although the plan was discarded by the British, Prime Minister Winston Churchill later advocated such a landing option. The so-called Ljubljana gap strategy proved ultimately to be little more than a bluff as a result of US refusal and scepticism about the whole operation. Nevertheless, the British command planned several landing operations in Dalmatia and Istria under the codename 'Armpit', and a more ambitious plan as 'Gelignite'. Facing US opposition, the British-made attempts were marked by sending the 'Fairfax' air unit to the Zadar area, and the 'Floyd' Force artillery attachment also to Dalmatia, but as a result of Yugoslav obstruction, such attempts then ceased. Nevertheless, the bluff worked since Hitler came to believe that an Allied landing in the northern part of the Adriatic Sea was likely, and therefore diverted important resources to the area. Instead of landings, the Allies agreed to provide Tito’s units with aerial and logistical support by the establishment of the Balkan Air Force.

The largest British-led combined operation in the eastern part of the Adriatic Sea, in December 1944, was 'Antagonise' to capture the island of Losinj, where the Germans kept Schnellboote and possibly midget submarines. The operation was only partially executed as the partisan navy’s commander, Josip Černi, refused to allocate troops for the landing operation. Instead, a group of British destroyers and motor torpedo boats shelled the German artillery positions and 36 Bristol Beaufighter twin-engined heavy fighter-bombers of the South African Air Force attacked the German naval installations on the island with 3-in (76.2-mm) rocket projectiles. The attacks proved ineffective in stopping German activities, so they were repeated in the first months of 1945.

By the end of October 1944, the Germans still had five torpedo boats (TA-20, TA-40, TA-41, TA-44 and TA-45) and three corvettes (UJ-205, UJ-206 and TA-48) in the Adriatic Sea. On 1 January 1945, there were four German destroyers (TA-40, TA-41 and TA-45) and three anti-submarine corvettes (UJ-205, UJ-206 and TA-48) operational in the northern part of the Adriatic Sea. As late as 1 April, TA-43, TA-45 and UJ-206 were still in commission and available to fight. Allied aircraft sank four of the German ships in port at Fiume and Trieste during March and April, and a British motor torpedo boat torpedoed TA-45 in April.

The very last German naval operations involved the evacuation of troops and personnel from Istria and Trieste before the arrival of the Yugoslav forces in May 1945. A German force estimated at 4,000 men was landed from 26 ships of all types at Lignano Sabbiadoro in the mouth of the Tagliamento river. This area is a very large sand spit extending into a big lagoon, and at its southern end the Tagliamento river enters the sea. The German evacuation force was protected by naval craft, which held back three British motor torpedo boats, which therefore could not close the range sufficiently to use their guns effectively. On the spit there were about 6,000 Germans in total and their equipment included Schnellboote, tank landing craft, one small hospital ship, several types of transport vessels, and a variety of weapons. The 21^st Battalion of the New Zealand 2^nd Division was outnumbered by 20/1, but the Germans nonetheless surrendered to it on 4 May. Other Germans had already surrendered to the British as they arrived at Ancona on as many as 30 vessels and craft from Istria on 2 May.