More than 75 years ago, the largest exchange of territory in occupied Germany led to drastic territorial changes in the Harz Mountains. The National Archive in London holds the documents that provide information about the reasons for the division of the Brunswick district of Blankenburg (Harz), which was formerly in the British zone. The reason for this was the British fears for the energy supply of their occupation zone, which were directly related to the Harbke coal-fired power station located in the Börde region on the demarcation line at the time.

The area west of an imaginary line Wismar - Magdeburg - Leipzig - Erzgebirge had been temporarily occupied by British and American units in April 1945. Thus the parts of Magdeburg province west of the Elbe were under the control of the 9th US Army, to which British Military Government Detachments were attached for the repatriation of displaced persons and the restoration of public order.
With the replacement of the 9th US Army by units of the British 21st Army Group, which began in mid-May 1945, the province of Magdeburg west of the Elbe came completely under British control. The British military government body responsible for the province of Magdeburg was placed under the command of the 229 /305 Provincial Military Government Detachment based in Hanover. As early as mid-May 1945, this military government responsible for the province of Hanover and the state of Braunschweig realised that the demarcation line agreed in Yalta between the British and Soviet occupation zones would mean considerable disadvantages for the economic viability of the British zone. The few undestroyed power plants in the British Zone, which were insufficient for energy supply, played a decisive role in this. In the first weekly report of the Hanoverian military government, for the period 10 - 17 May 1945, the provision of electrical energy for the British Zone was described. It is worth noting here that 65% of the available energy in the province of Magdeburg was produced in the Harbke power station. In the third weekly report, the British even assumed that in the medium term large amounts of electrical energy would also be transmitted from the Leipzig area to the British Zone.
The withdrawal of US troops from the temporarily occupied territories in the Soviet zone announced by US President Truman on 11 June 1945 was therefore all the more sobering for the British. At the urging of the Soviets, who wanted to take complete control of their zone of occupation after the Allied declaration to assume supreme power in Germany, the Americans had taken the British completely by surprise with the early withdrawal of troops.
This prompted the military government in Hanover to submit a bold proposal to the High Command of the 21st British Army Group on 15 June 1945 to extend the British zone eastwards. For reasons of assumed affiliation to the economic area of Lower Saxony, parts of the districts of Gardelegen, Haldensleben, Oschersleben, the entire district of Wernigerode, the city of Halberstadt, parts of the district of Quedlinburg including the district town and parts of the Thuringian district of Grafschaft Hohenstein (Nordhausen) were to be incorporated into the British Zone.
With the agreed withdrawal of British and US troops from the temporarily occupied territories, which was completed on 5 July 1945, such a proposal, which would have secured the British the energy supply from the Harbke power station as well, would have been tantamount to an affront to the Soviets. In order to nevertheless gain control and possession of the all-important Harbke power station, the Commanding General of the British 30th Corps initiated an alternative proposal on 7 July 1945. By negotiation, the Soviets were to be persuaded to cede the Harbke power station and parts of the county of Hohenstein (Bad Sachsa and Tettenborn) to the British zone in exchange for large parts of the Brunswick district of Blankenburg.
Since 10 July 1945, the British, under the leadership of the 21st Army Group, had been negotiating with the leadership of the 125th Guards Rifle Corps of the Red Army. In the negotiations, however, the Soviets insisted that the Harbke power station remain in their zone. The British and the Soviets, each represented by the corps commanders, agreed on a permanent supply of 75% of the power plant's energy output, in exchange for coal supplies from the Helmstedt coalfield in the British zone. In exchange for the eastern part of the Blankenburg district, the British demanded that the Soviets take over Bad Sachsa and Tettenborn in order to significantly shorten the demarcation line.
On 18 July 1945, the 21st Army Group authorised the Commanding General of the 30th Corps to ratify the negotiated agreement for the exchange of territories between the British and Soviet occupation zones. The date for the exchange of territory was set for 23 July 1945, 08:00 local time. Until then, the British took extensive measures to withdraw the 111th District Military Government Detachment responsible for the Blankenburg district and parts of their 5th Infantry Division from the area to be ceded by 22 July 1945.

In order to be able to incorporate the agreed changes to the demarcation line into the results of the Potsdam Agreement, Field Marshal Montgomery, the British representative on the Allied Control Council for Germany, introduced a memorandum to this effect on 30 July 1945, the day of the constituent meeting of the Control Council. Both from the text of the illustrated memorandum and from the map extract of the illustrated Annex C to this memorandum, an apparent advantage of the Soviets in the underlying negotiations can be assumed. With the cession of the eastern part of the Blankenburg district, it is estimated that more than 30,000 people were transferred to the custody of the Soviets, with a simultaneous transfer of an estimated 6,000 people to the British Zone.
The actual significance of the memorandum for the British, however, does not emerge directly from it. Although a significant shortening of the demarcation line for the British can be deduced from it, the decisive regulation for the energy supply of the British Zone and thus the actual British intention remains unmentioned.
With the Harbke Agreement, concluded later in October 1945, the British secured electricity supplies in exchange for the supply of lignite from the Helmstedt coalfield until 1952.
The exchange of territories was not reversed after the reunification of Germany in 1990.