'Regenbogen' (ii) was a German undertaking schemed by Grossadmiral Karl Dönitz, Adolf Hitler’s successor as the German leader and commander-in-chief of the German navy, for the scuttling of all warships and U-boats at the end of World War II in Europe (30 April/4 May 1945).

By the end of April 1945, Germany was collapsing under the Allied onslaughts from east and west. Soviet forces had laid siege to Berlin, and on 30 April Hitler had committed suicide. He had appointed Dönitz as head of state and supreme commander of the German armed forces. US forces from the west and Soviet forces from the east had already met at Torgau on the Elbe river, cutting what was left of Germany into northern and southern areas, while in the north the Anglo-Canadian 21^st Army Group was poised to capture Hamburg and the other German ports on the North Sea and western end of the Baltic Sea.

The state of the German navy was now very poor. Of its major surface warships, only the heavy cruiser Prinz Eugen still survived, sheltering at Copenhagen, and only the U-boat arm was capable of continuing the fight. The Kriegsmarine had about 470 U-boats remaining, and of these some 170 were operational. These boats were based mainly in occupied Norway, and another 200 boats, in various stages of construction, commissioning and working up, were located mainly in the north German ports and in Baltic ports.

As head of the navy and commander of the U-boat arm, Dönitz was adamant that his U-boat force should not be surrendered. However, as the new German leader he was desperate to extricate Germany from the war and, if possible, avoid Allied, and especially Soviet, retribution. To that end he had opened negotiations with the western Allies, through Field Marshal Sir Bernard Montgomery, commander of 21^st Army Group in northern Germany.

As the Allies closed on the north German ports, the Kriegsmarine started to destroy what was left of it to prevent its capture, while all serviceable boats were ordered to bases in Norway. During May a final massacre of U-boats fleeing to Norway took place, some 23 U-boats being destroyed or damaged beyond repair in transit during the first week of May.

Against this backdrop, Dönitz and the U-boat arm laid plans for a mass scuttling of the U-boats, to be carried out on receiving the code-word 'Regenbogen'.

During May the Kriegsmarine started to scuttle its U-boats ahead of the advancing Allied armies. On 1 May, three boats were wrecked at Warnemünde, outside Rostock on the Baltic Sea coast, the first of the wave of boat scuttlings and associated facilities destruction. On 2 May, a further 32 boats were scuttled at Travemünde near Lübeck. On 3 May, Dönitz sent his chief aide, Generaladmiral Hans-Georg von Friedeburg, to Montgomery at Lüneburg to open negotiations for an armistice with the western Allies. This was refused as von Friedeburg was not empowered to agree an unconditional surrender of the type which the Allies demanded. Also on 3 May, another 39 boats were wrecked, 32 at Kiel and another seven at Hamburg, on the North Sea coast.

On 4 May the Allied supreme commander in Europe, General Dwight D. Eisenhower, allowed the German forces in North-West Europe, including naval forces, to surrender to Montgomery and the 21^st Army Group. For this, Montgomery insisted that German naval forces, including the U-boat arm, be surrendered intact. This surrender was to become effective at 08.00 on 5 May. Meanwhile, on 4 May, four more boats, two in the Kiel Canal and two at Flensburg, were scuttled.

In the early hours of 5 May, the 'Regenbogen' order was given, only to be countermanded 8 minutes later to avoid jeopardising the surrender negotiations, and later that day all operational U-boats were ordered to cease hostilities. Despite this a further 87 boats were destroyed on 5 May; 64 in the Baltic (41 at Gelting Bay, 13 at Flensburg and 10 at various other points), while on the North Sea coast 23 boats were scuttled, 13 of them at Wilhelmshaven and 10 in the Weser river estuary.

On 6 May, there were no further sinkings, but on 7 May the two advanced boats with the Walter high-test peroxide propulsion system, were wrecked at Cuxhaven. Over these last seven days, at least 195 U-boats had been scuttled.

On 8 May, Germany surrendered unconditionally, and the surviving naval units, including U-boats, surrendered to Allied forces. At least 150 U-boats were surrendered to the Allied navies, either at sea or at their operational bases: 52 boats were surrendered at sea, either on patrol or in transit, and 98 in port, mostly in Norway and at bases in Germany, Denmark and France. Two U-boats (U-1277 and U-963) fled to Portuguese waters, where they were scuttled by their crews, the former off Porto and the later some miles from Nazaré. Two others (U-530 and U-977) arrived in Mar del Plata in Argentina, where their crews surrendered to the local authorities.

Various figures have been claimed for the numbers of U-boats involved during this period: some of boats were not currently in commission, some had not yet been commissioned, and some had been decommissioned, and the discrepancies in the overall total largely reflect whether or not these are included. Most sources give a number of boats scuttled at the end of the war and describe the 'Regenbogen' order, conflating the two. What is debatable, however, is the extent to which the order was implemented, or indeed whether or not it was actually given.