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For the most part, the campaign was fought between the Italian Royal Navy (Regia Marina), supported by other Axis naval and air forces, and the British Royal Navy, supported by other Allied naval forces, such as Australia, the Netherlands, Poland and Greece. American naval and air units joined the Allied side in 1942.
Each side had three overall objectives in this battle. The first was to attack the supply lines of the other side. The second was to keep open the supply lines to their own armies in North Africa. The third was to destroy the ability of the opposing navy to wage war at sea. Outside of the Pacific theatre, the Mediterranean saw the largest conventional naval warfare actions during the conflict. In particular, Allied forces struggled to supply and retain the key naval and air base of Malta.
By the time of the September 1943 armistice between Italy and the Allies, Italian ships and aircraft had sunk Allied surface warships totaling 145,800 tons, while the Germans had sunk 169,700 tons, for a total of 315,500 tons. In total the Allies lost 76 warships and 46 submarines. The Allies sank 83 Italian warships totaling 195,100 tons (161,200 by the Commonwealth and 33,900 by the Americans) and 83 submarines. German losses in the Mediterranean from the start of the campaign to the end were 68 submarines.
The Mediterranean was a traditional focus of British maritime power. Outnumbered by the forces of the Regia Marina, the British plan was to hold the three decisive strategic points of Gibraltar, Malta, and the Suez Canal. By holding these points, the Mediterranean Fleet held open vital supply routes. Malta was the lynch-pin of the whole system. It provided a needed stop for Allied convoys and a base from which to attack the Axis supply routes.
Italian dictator Benito Mussolini saw the control of the Mediterranean as an essential prerequisite for expanding his "New Roman Empire" into Nice, Corsica, Tunis and the Balkans. Italian naval building accelerated during his tenure. Mussolini described the Mediterranean Sea as Mare Nostrum "(our sea)".
The warships of the Regia Marina (Italian Royal Fleet) had a general reputation as well-designed. Italian small attack craft lived up to expectations and were responsible for many brave and successful actions in the Mediterranean. But some Italian cruiser classes were rather deficient in armour and all Italian warships lacked radar although its lack was partly offset by Italian warships being equipped with good rangefinder and fire-control systems for daylight combat. Only by the spring of 1943, barely five months before the armistice, twelve Italian warships were equipped with Italian-designed EC-3 ter Gufo radar devices. In addition, whereas Allied commanders at sea had discretion to act on their own initiative, the actions of Italian commanders were closely and precisely governed by Italian Naval Headquarters (Supermarina).
The Regia Marina also lacked a proper fleet air arm. The aircraft carrier Aquila was never completed and most air support during the Battle of the Mediterranean was supplied by the land-based Regia Aeronautica (Royal Air Force). Another major handicap for the Italians was the shortage of fuel. As early as March 1941, the overall scarcity of fuel oil was critical. Coal, gasoline and lubricants were also locally hard to find. During the Italian war effort, 75% of all the fuel oil available was used by destroyers and torpedo boats carrying out escort missions.
However, the most serious problem for the Axis forces in North Africa was the limited capacity of the Libyan ports. Even under the best conditions, this restricted supplies. Tripoli was the largest port in Libya and it could accommodate a maximum of five large cargo vessels or four troop transports. On a monthly basis, Tripoli had an unloading capacity of 45,000 short tons (41,000 t). Tobruk added only another 18,000 short tons (16,000 t). Bardia and other smaller ports added a little more.
In general, the Axis forces in North Africa exceeded the capacity of the ports to supply them. It has been calculated that the average Axis division required 10,000 short tons (9,100 t) of supplies per month. If the Italians had a fault in respect to logistics during the Battle of the Mediterranean, it was that they failed to increase the capacity of Tripoli and the other ports before the war.
In January 1937, France began a programme of modernisation and expansion. This soon elevated the French Fleet to the fourth-largest in the world. However, the French Navy (formally the "National Navy" - Marine Nationale), was still considerably smaller than the navy of its ally, Britain.
By agreement with the British Admiralty, the strongest concentration of French vessels was in the Mediterranean. Here, the Italian Fleet posed a threat to the vitally important French sea routes from metropolitan France to North Africa and to the British sea routes between Gibraltar and the Suez Canal.
In 1940, after France fell to the Germans, the Marine Nationale in the Mediterranean became the navy of the Vichy French government. As the Vichy French Navy, this force was considered a potentially grave threat to the Royal Navy. As such, it was imperative to the British that this threat be neutralised.
As the opening phase of Operation Catapult, the French squadron at Alexandria in Egypt was dealt-with via negotiations. This proved possible primarily because the two commanders—Admirals René-Emile Godfroy and Andrew Cunningham—were on good personal terms. In contrast, a British ultimatum to place the bulk of the remainder of the French fleet out of German reach was refused. The fleet was located at Mers-el-Kebir in Algeria, so on 3 July 1940 it was largely destroyed by bombardment by the British "Force H" from Gibraltar (Admiral James Somerville). The Vichy French government broke off all ties with the British as a result of this attack and the Vichy French Air Force (Armée de l'Air de Vichy) even raided British installations at Gibraltar.
In June and July 1941, a small Vichy French naval force was involved during "Operation Exporter". This was an Allied action launched against Vichy French forces based in Lebanon and Syria. French naval vessels had to be driven off before the Litani River could be crossed.
In 1942, as part of the occupation of Vichy France during "Case Anton", the Germans intended to capture the French fleet at Toulon. This was thwarted by determined action by French commanders; the bulk of the fleet was scuttled at anchor.
The Mediterranean U-boat Campaign lasted approximately from 21 September 1941 to May 1944. Germany's Kriegsmarine aimed at isolating Gibraltar, Malta, and the Suez Canal so as to break Britain's trade route to the far east. More than 60 U-boats were sent to disrupt shipping in the sea, although many were attacked in the Strait of Gibraltar, which was controlled by Britain (nine boats were sunk while attempting the passage and ten more were damaged). The Luftwaffe also played a key part in the Battle of the Mediterranean, especially during 1941. German war strategy, however, viewed the Mediterranean as a secondary theatre of operations.
On 10 June 1940, Italy declared war on Britain and France. On the following day, Italian bombers attacked Malta on what was to be the first of many raids. During this time, the Marine Nationale shelled a number of targets on the northwestern coast of Italy, in particular the port of Genoa. When France surrendered on 24 June, the Axis leaders allowed the new Vichy French regime to retain its naval forces.
The first clash between the rival fleets—the Battle of Calabria—took place on 9 July, just four weeks after the start of hostilities. This was inconclusive, and was followed by a series of small surface actions during the summer, among them the battle of the Espero convoy and the battle of Cape Spada.
To reduce the threat posed by the Italian fleet, which was based in the port of Taranto, to convoys sailing to Malta, Admiral Cunningham organised an attack code-named Operation Judgement. Fairey Swordfish torpedo bombers from HMS Illustrious attacked the Italian fleet on 11 November 1940 while it was still at anchor. This was the first time that an attack such as this had been attempted and it was studied by Japanese naval officers in preparation for the later attack on Pearl Harbor. British Fleet Air Arm aircraft badly damaged two Italian battleships and a third was sunk putting half of the regia Marina's major ships out of action for several months. This attack also forced the Italian fleet to move to Italian ports further north so as to be out of range of carrier-based aircraft. This reduced the threat of Italian sallies attacking Malta-bound convoys.
Cunningham's estimate that Italians would be unwilling to risk their remaining heavy units was quickly proven wrong. Only five days after Taranto, Inigo Campioni sortied with two battleships, six cruisers and 14 destroyers to disrupt a British aircraft delivery operation to Malta.
Furthermore, as early as 27 November, the Italian fleet was able to confront the Mediterranean fleet again in the indecisive battle of Spartivento. Two of the three damaged battleships were repaired by mid-1941 and control of the Mediterranean continued to swing back and forth until the Italian armistice in 1943. Measured against its primary task of disrupting Axis convoys to Africa, the Taranto attack had little effect. In fact, Italian shipping to Libya increased between the months of October 1940–January 1941 to an average of 49,435 tons per month, up from the 37,204-ton average of the previous four months. Moreover, rather than change the balance of power in the central Mediterranean, British naval authorities had "failed to deliver the true knockout blow that would have changed the context within which the rest of the war in the Mediterranean was fought." 
The Battle of Cape Matapan was a decisive Allied victory. It was fought off the coast of the Peloponnese in southern Greece from 27–29 March 1941 in which Royal Navy and Royal Australian Navy forces—under the command of the British Admiral Andrew Cunningham—intercepted those of the Italian Regia Marina under Admiral Angelo Iachino.
The Allies sank the heavy cruisers Fiume, Zara and Pola and the destroyers Vittorio Alfieri and Giosue Carducci, and damaged the battleship Vittorio Veneto. The British lost one torpedo plane and suffered light splinter damage to some cruisers from Vittorio Veneto's salvoes.
The decisive factors in the Allied victory were the effectiveness of aircraft carriers, the use of Ultra intercepts and the lack of radar on the Italian ships.
The effort to prevent German troops from reaching Crete by sea, and subsequently the partial evacuation of Allied land forces after their defeat by German paratroops in the Battle of Crete during May 1941, cost the Allied navies a number of ships. Attacks by German planes, mainly Junkers Ju 87s and Ju 88s, sank eight British warships: two light cruisers (HMS Gloucester and Fiji) and six destroyers (HMS Kelly, Greyhound, Kashmir, Hereward, Imperial and Juno). Seven other ships were damaged, including the battleships HMS Warspite and Valiant and the light cruiser Orion. Nearly 2,000 British sailors died.
It was a significant victory for the Luftwaffe, as it proved that the Royal Navy could not operate in waters where the German Air Force had air supremacy without suffering severe losses. In the end, however, this had little strategic meaning, since the attention of the German Army was directed toward Russia (in Operation Barbarossa) a few weeks later, and the Mediterranean was to play only a secondary role in German war planning over the following years. The action did, however, extend the Axis reach into the eastern Mediterranean, and prolong the threat to Allied convoys.
Two attempts were carried out to transport German troops by sea in caïques, but both of them were disrupted by Royal Navy intervention. The tiny Italian naval escorts, however, managed to save most of the vessels. Eventually, the Italians landed a force of their own near Sitia on 28 May, when the Allied withdrawal was already ongoing.
During the evacuation, Cunningham was determined that the "Navy must not let the Army down". When army generals stated their fears that he would lose too many ships, Cunningham said that "It takes three years to build a ship, it takes three centuries to build a tradition". Despite advance warning through Ultra intercepts, the Battle of Crete resulted in a decisive defeat for the Allies. The invasion took a fearful toll of the German paratroops, who were dropped without their major weapons, which were delivered separately in containers. So heavy were the losses that General Kurt Student, who commanded the German invasion, would later say, referring to the German decision not to use parachutists in any future invasion attempts:
"Crete was the grave of the German parachutists."
Malta's position between Sicily and North Africa was perfect to interdict Axis supply convoys destined for North Africa. It could thus influence the campaign in North Africa and support Allied actions against Italy. The Axis recognised this and made great efforts to neutralise the island as a British base, either by air attacks or by starving it of its own supplies.
For a time during the Siege of Malta, it looked as if the island would be starved into submission by the use of Axis aircraft and warships based in Sicily, Sardinia, Crete and North Africa. A number of Allied convoys were decimated. The turning point in the siege came in August 1942, when the British sent a very heavily defended convoy under the codename Operation Pedestal. Malta's air defence was repeatedly reinforced by Hawker Hurricane and Supermarine Spitfire fighters flown to the island from HMS Furious and other Allied aircraft carriers. The situation eased as Axis forces were forced away from their North African bases and eventually Malta could be resupplied and become an offensive base once again.
The British re-established a substantial air garrison and offensive naval base on the island. With the aid of Ultra, Malta's garrison was able to disrupt Axis supplies to North Africa immediately before the Second Battle of El Alamein. For the fortitude and courage of the Maltese people during the siege, the island was awarded the George Cross.
The Royal Navy sank 3,082 Axis merchantmen in the Mediterranean, amounting to over 4 million tons. The loss of supplies proved fatal to the Axis armies in North Africa.
Following the battle of Crete in the summer of 1941, the Royal Navy regained its ascendancy in the central Mediterranean in a series of successful convoy attacks, (including the Duisburg convoy and Cap Bon), until the events surrounding the First Battle of Sirte and the Raid on Alexandria in December swung the balance of power towards the Axis.
The Regia Marina's most successful attack was when divers attached limpet mines on the hulls of British battleships during the Raid on Alexandria on 19 December 1941. The battleships HMS Queen Elizabeth and HMS Valiant were sunk at their berths, but they were both later raised and returned to active service.
A series of hard fought convoy battles (such as the Second Battle of Sirte in March, Operations Harpoon and Vigorous in June and Operation Pedestal in August), ensured Malta's survival until the Allies regained the advantage in November 1942.
In September 1943, with the Italian collapse and the surrender of the Italian fleet, naval actions in the Mediterranean became restricted to operations against U-boats and by small craft in the Adriatic and Aegean seas.
On 25 July 1943, the Grand Council of Fascism ousted Mussolini. A new Italian government, led by King Victor Emmanuel III and Marshal Pietro Badoglio, immediately began secret negotiations with the Allies to end the fighting and to come over to the Allied side. On 3 September, a secret armistice was signed with the Allies at Fairfield Camp in Sicily. The armistice was announced on 8 September.
After the armistice, the Italian Navy was split in two. In southern Italy, the "Co-Belligerent Navy of the South" (Marina Cobelligerante del Sud) fought for the King and Badoglio. In the north, a much smaller portion of the Regia Marina joined the Republican National Navy (Marina Nazionale Repubblicana) of Mussolini's new Italian Social Republic (Repubblica Sociale Italiana, or RSI) and fought on for the Germans.