'Swamp' was the Allied overall designation of all-out search and destroy operations against Axis submarines in the Mediterranean Sea (autumn/winter 1943/44).

In any such operation the detected submarine was kept submerged until it finally had to surface for air, and was then overwhelmed by the attacks of Allied warships.

The Mediterranean U-boat campaign lasted from 21 September 1941 to 19 September 1944. The Italians having failed to neutralise Malta as a British base, the Axis supply convoys to North Africa were suffering severe losses, and this in turn threatened the Axis armies' ability to fight effectively in the Western Desert campaign as a time when the Allies were able to maintain the flown of men, weapons, equipment and supplies to their forces in this theatre and, to a lesser and often threatened degree, to the island bastion of Malta. In light of the failure of the Italian submarine and bomber efforts to do so, the Kriegsmarine was entrusted with the task of isolating Malta so as to disrupt British supply convoys to the island and, if possible, force it to surrender for lack of food. As the Allies gained the upper hand, U-boat operations became targeted at the various landings in southern Europe.

Some 60 U-boats made the difficult passage into the Mediterranean Sea via the British-dominated Strait of Gibraltar, and of these only one managed to make the passage back into the Atlantic Ocean. Admiral Karl Dönitz, the Befehlshaber der Unterseeboote was steadily reluctant to send his boats into the Mediterranean, as he saw the Atlantic as the primary theatre, but recognised that natural chokepoints such as the Strait of Gibraltar were more likely to result in vulnerable concentrations of shipping.

Of the U-boats sent to support the Italian effort, many were attacked in the Strait of Gibraltar, losing nine of their number sunk and another 10 damaged. Had these U-boats been deployed in the Atlantic, or directly along the coasts of the UK, it can be argued that Germany would have been offered a greater advantage.

The German navy already possessed some knowledge of the area, for U-boats had operated with great success in the Mediterranean during World War I (Dönitz himself had been an officer on UB-68, which had been sunk in the region), and U-boats had also supported the Nationalist insurgent faction in the Spanish Civil War (1936/39). The Republican government’s navy had 12 submarines, while the Nationalists had none and therefore welcomed the advent of U-boats. The first two of the boats were U-33 and U-34 which, under the codename 'Ursula', departed Wilhelmshaven on 20 November 1936. Both boats sailed down the English Channel and entered the Mediterranean on the night of 27 November. The boats were soon in action, U-34 launching a single torpedo at a Republican destroyer in the evening of 1 December. The projectile missed, impacting on rocks. Under the command of Leutnant Harald Grosse, the boat made other attacks on 5 and 8 December, but again gained no success. U-33 did no better, its commander being frustrated by the absence of target identification or defensive movement of his intended victims. The only vessel was sunk by the U-boats was the Republican submarine C-3, which was torpedoed and sunk by U-34 on 12 December.

By October 1939, Dönitz had decided to use three longer-range boats to intercept the first Allied convoys of the war. U-25, U-26 and U-53 met in an area to the south-west of Ireland before attempting to force the Strait of Gibraltar and attack Allied convoys in the Mediterranean, but matters seemed to be inauspicious right from the start when U-25 was diverted against a convoy to the south-west of Lisbon in Portugal. After an unsuccessful torpedo attack on a steamer on 31 October, Kapitänleutnant Viktor Schütze, U-25‍ '​s commander, surfaced and sank his target with fire from his deck gun. This caused a crack in a vital part of the boat, which compelled it to return to Germany.

U-53 ran short of fuel after shadowing a convoy in the Bay of Biscay and was also forced to return, which left only U-26 available to enter the Mediterranean. A combination of adverse weather, searchlights and British anti-submarine patrols forced Kapitänleutnant Ernst-Günter Heinicke, the boat’s commander, to abandoned an attempt to mine the waters off Gibraltar, and the boat passed through the Strait of Gibraltar on the surface and claimed only one ship sunk in the Mediterranean: this claimed was not confirmed by post-war analysis. U-26 headed back through the Strait of Gibraltar and arrived back in Wilhelshaven on 5 December 1939 as the sole U-boat to enter and then leave the Mediterranean in World War II.

The 23^rd Unterseeboots-Flottille was established in September 1941 under the command of Kapitänleutnant Fritz Frauenheim with its base at Salamis in occupied Greece, to intercept coastal shipping being used to sustain the Allied forces through the siege of Tobruk, and U-boats patrolled the eastern basin of the Mediterranean. On 7 December control of the 23^rd Unterseeboots-Flottille was transferred from Kernével in the north-western part of German-occupied France, where Dönitz had his headquarters, to the German command in Italy, headed by Generalfeldmarschall Albert Kesselring. Additional bases were established in Pula on the Croat coast of occupied Yugoslavia and La Spezia in north-western Italy as more U-boats were ordered to the Mediterranean until the German navy’s focus shifted to the western Atlantic through the 'second happy time'. Up to this time 29 U-boats had entered the Mediterranean.

La Spezia became the headquarters when the boats on the Mediterranean were reorganised as Korvettenkapitän Fritz Frauenheim’s 29^th Unterseeboots-Flottille in May 1942. No more U-boats were assigned to the Mediterranean from mid-January to early October 1942 as opportunities along the east coast of North America seemed more productive during the 'second happy time', and Generaloberst Erwin Rommel’s Panzerarmee 'Afrika' was advancing toward Egypt. The 29^th Unterseeboots-Flottille centred its attentions on the Allied convoys nourishing Malta and the British forces on the Egyptian/Libyan coast. For sustained operations, U-boats spent approximately one-third of their time on patrol, one-third in transit to and from base for routine provisioning and refuelling, and one-third undergoing major overhaul or repair. The 29^th Unterseeboots-Flottille's strength of 20 boats enabled a routine patrol strength of three U-boats from Salamis in the eastern basin of the Mediterranean, and three from La Spezia in the western basin of the Mediterranean. The loss of U-372 and U-568 in 12-hour sustained attacks demonstrated vulnerability of single boats to a team of destroyers which could hunt a submerged boat to exhaustion of air and battery power, rather than moving on after a few attacks.

More boats were assigned to the 29^th Unterseeboots-Flottille after the Allies' improvement of anti-submarine capability along the east coast of North America ended the 'second happy time'.

When a patrolling Short Sunderland flying boat located U-559, it summoned five destroyers able to maintain contact while dropping 150 depth charges over a period of 10 hours until the submarine attempted to sneak away on the surface at night. Waiting destroyers open fire as soon as the boat surfaced, and the crew abandoned its vessel. The Royal Navy boarded the sinking boat and recovered German code documents before U-559 sank.

The 2^nd Battle of El Alamein prompted a concentration of boats in the western basis of the Mediterranean in anticipation of an Allied amphibious invasion. Five U-boats made contact with the 'Torch' convoys, and two wolfpacks assembled near the invasion points. U-73, U-81, U-458, U-565, U-593, U-595, U-605 and U-617 constituted 'Delphin' (i) around Oran, and U-77, U-205, U-331, U-431, U-561 and U-660 constituted 'Hai' around Algiers. The Germans lost five boats in their efforts to oppose the 'Torch' invasion.

The Allied armies advancing through North Africa and Sicily constructed a system of airfields, increasing the frequency at which the boats were detected from the air. The 29^th Unterseeboots-Flottille focused its efforts on the convoys in the western basin of the Mediterranean supplying Allied troops, but three U-boats were based at Salamis to maintain a patrol presence in the eastern basin of the Mediterranean, and thus make its necessary for the Allies to dilute its anti-submarine effort. On 1 August 1943 the 29^th Unterseeboots-Flottille shifted its headquarters from La Spezia to Toulon, where it could use the former French naval base for patrols in the western basin of the Mediterranean.

In the time after Italy’s capitulation in September 1943, the Allied escort forces in the Mediterranean became more numerous, and their tactic of hunting a detected U-boat to exhaustion was given the name 'Swamp' and used with increasing frequency. The boats used the new G7e passive-homing torpedo against destroyers, but were unable to cope with a team of escorts. U-boats remaining in port were subjected to Allied bombing raids from newly constructed airfields. The surviving boats of the 29^th Unterseeboots-Flottille were all scuttled at Toulon when the 'Dragoon' (i) invasion of southern France closed the base on 15 August 1944. The three boats remained at Salamis until Allied forces reached the island on 19 September 1944.

In overall terms, the 62 boats which operated in the Mediterranean sank 95 Allied merchant ships totalling 449,206 tons and 24 Royal Navy warships including two carriers, one battleship, four cruisers and 12 destroyers. All the boats were themselves lost or scuttled.

Four U-boats were sunk by Allied submarines in the Mediterranean.