On the night of 17/18 August 1943, RAF bombers carried out Operation Hydra against the Peenemünde Army Research Center where V-2 development and production was being carried out.
On 19 October 1943, the German limited company Mittelwerk GmbH was issued War Contract No. 0011-5565/43 by General Emil Leeb, head of the Army Weapons Office, for 12,000 A-4 missiles at 40,000 Reichsmarks each.
Mittelwerk GmbH also headed sites for V-2 rocket development and testing at Schlier (Project Zement) and Lehesten. Beginning in May 1944, Georg Rickhey was the Mittelwerk general manager, Albin Sawatzki was the Mittelwerk technical director over both Arthur Rudolph's Technical Division (with deputy Karl Seidenstuecker) and Hans Lindenberg's 50 engineers of the quality control group located at Ilfeld. Other Mittelwerk/Ilfield engineers included Magnus von Braun in turbopump production, Guenther Haukohl who supervised V-2 production after helping design the assembly line, Eric Ball (assembly line), Hans Fridrich, Hans Palaoro and Rudolph Schlidt. The facility had a communications staff under Captain Dr Kühle, an Administrative Division run by Börner under Mittelwerk board member Otto Karl Bersch, and a Prisoner Labor Supply office (Brozsat). Hannelore Bannasch was Sawatzki's secretary.
In July 1944, Hans Kammler ordered the North Works (Nordwerke) to use cross-tunnels 1-20 for a Junkers jet and piston engine factory, leaving cross-tunnels 21-46 for Mittelwerk GmbH. During February–April 1945, the Nordhausen plant built Taifun anti-aircraft missiles and Heinkel He 162 jet fighters and put into operation a liquid oxygen plant. The plant was the Eber project and used equipment evacuated from the Watten bunker and elsewhere to build Heylandt liquid oxygen generators; the 15 generators were nearly complete when the site was captured. The Mittelwerk also contained equipment for producing jet fuel, and in an emergency 1944 decentralization program (named Geilenbergprogramm after Edmund Geilenberg) started the "Cuckoo" project, an underground oil plant to be "carved out of the Himmelsburg" North of the Mittelwerk.
Additional plans for V-2 rocket plants (the Southern Works near Friedrichshafen and the Eastern Works near Riga) were never fulfilled.
V-1 flying bomb assembly began during October/November 1944 in the South end of tunnel A. At the end of January 1945, 51 V-1s were shipped from a dispersed Fieseler factory in Upper Bavaria (code name Cham) to the Nordhausen plant for completion. After a second V-1 factory at Burg was closed, the Mittelwerk Werk II in February 1945 was the only factory producing V-1 flying bombs, and a total of 2,275 V-1s were built by Werk II from September 1944 until April 1945.
Although there has long been speculation about other "exotic" weaponry being constructed or stored at Mittelwerk, evidence of this is scarce. For example, Richard Overy notes in The Bombing War - Europe 1939-1945 (2013): "There is some evidence that small spherical bombs containing radioactive waste were stored in the Mittelbau-Dora works [...], but it is not conclusive."
In late February 1945, the Allied Chiefs of Staff discussed a proposed attack on the Nordhausen plant with a highly flammable petroleum-soap mixture that had been used in the Pacific theatre to deeply penetrate buried strongpoints and scourge them with intense heat. The area was attacked with conventional bombs by RAF Bomber Command on the 3 & 4 April. What were believed to be barracks were attacked on 3 April but they actually contained forced labor workers. The attack of 4 April hit the barracks and the town of Nordhausen. The Mittelbau-Dora forced labor was evacuated on 4 April, and scientists evacuated to the Alpenfestung (English: Alpine Fortress). Hitler had made an order, the "Demolitions on Reich Territory Decree", which ordered the destruction of any infrastructure that might be of use to the Allies but it was deliberately ignored by Speer and the Nordhausen plant was evacuated without damage.
Having been warned to "expect something a little unusual in the Nordhausen area", and after previously entering the Nordhausen plant from the North through the Junkers Nordwerke, U.S. 3rd Armored Division and 104th Infantry Divisions reached the city of Nordhausen on 11 April 1945 and discovered the dead and sick of the Boelcke Kaserne barracks.
Casualties of the V-2 rocket are estimated at 2,541 killed and 5,923 injured. By contrast, of the roughly 60,000 people who passed through Mittelbau-Dora and its subcamps, an estimated 20,000 died either at the camp or at places they were subsequently transported to: 350 were hanged (including 200 for sabotage), many others died from exhaustion, cold, malnutrition or disease. Some were murdered by guards. The total also includes 1,300 to 1,500 prisoners killed by British bombs in early April.
On 22 May 1945, US Army Special Mission V-2 shipped the first trainload of rocket parts for use in projects such as Operation Sandy, Operation Blossom and, at the White Sands Proving Grounds, the Hermes project. The Nordhausen area was to become part of the Soviet zone of occupation, and Soviet Army officers arrived to tour the Nordhausen plant on 26 May 1945. In June 1945, the US Army left the Nordhausen plant as required by JCS Directive 1067/14, with parts, machine tools, and documents (including blueprints for the projected A-9/A-10 intercontinental missile) left for the Soviets. The Red Army occupied the Mittelwerk on 5 July 1945 and demolished both of the entrances of the tunnel system in the summer of 1948.
The 1947 Dora Trial convicted SS Officers and concentration camp kapos, while 3 scientists of the V-2 rocket program were implicated (2 after the trial) and exonerated of Nazi war crimes at the Mittelwerk. On 19 May 1947, the former head of the Mittelwerk facility, Georg Rickhey, was extradited to Germany from Wright Field in the U.S. and acquitted of war crimes at the Dora Trial. Arthur Rudolph, after immigrating to the U.S. and playing key roles in the Pershing missile and Apollo programs, was forced to renounce his U.S. citizenship and return to Germany; the West German government, citing the statute of limitations, never charged him and eventually granted him citizenship. Wernher von Braun, the Technical Director of a separate facility at Peenemünde Army Research Center, visited the Mittelwerk on 25 January 1944, and a 1991 author alleged he witnessed Mittelwerk and Buchenwald war crimes.
After a new entrance tunnel had been dug to former rail Tunnel A in 1995, 710 meters of the tunnel system were opened for visitors. Large parts of the system are flooded by ground water, while other parts have collapsed. After the reunification of Germany the tunnels were frequently looted by treasure seekers who gained access via the private mine in the north of the Kohnstein.
Willi Kramer, a German archaeologist and scientist who dived in the tunnel system in 1992 and 1998, estimated that 70 tons of material was stolen. Access through these entrances was not secured until 2004, when the mine went into insolvency.