Herzogtum Lauenburg is the southernmost Kreis, or district, of Schleswig-Holstein, Germany. It is bordered by (from the west and clockwise) the district of Stormarn, the city of Lübeck, the state of Mecklenburg-Vorpommern (districts of Nordwestmecklenburg and Ludwigslust-Parchim), the state of Lower Saxony (districts of Lüneburg and Harburg), and the city state of Hamburg. The district of Herzogtum Lauenburg is named after the former Duchy of Saxe-Lauenburg.
The district Herzogtum Lauenburg is named after the medieval Duchy of Saxe-Lauenburg, which was one of the remnants of the original Duchy of Saxony. The Duchy of Saxony was partitioned in a process started in 1269, nine years after in 1260 Albert II and John I had succeeded their father Albert I of Saxony. In 1269, 1272 and 1282, the brothers gradually divided their governing competences within the three territorially unconnected Saxon areas along the Elbe river (one called Land of Hadeln, another around Lauenburg upon Elbe and the third around Wittenberg upon Elbe), thus preparing a partition.
After John I's resignation in 1282, Albert II ruled with his still minor nephews Albert III, Eric I and John II, who by 1296 definitely partitioned the duchy providing Saxe-Lauenburg for the brothers, and Saxe-Wittenberg for their uncle Albert II. The last document, mentioning the brothers and their uncle Albert II as Saxon fellow dukes dates back to 1295. A deed of 20 September 1296, circumscribing Saxe-Lauenburg, mentions the Vierlande (now Hamburg), Sadelbande (Land of Lauenburg), the Land of Ratzeburg, the Land of Darzing (later Amt Neuhaus), and the Land of Hadeln (the latter two now Lower Saxony) as the joint territory of the brothers, separate of Saxe-Wittenberg.
Saxe-Lauenburg was also known simply as Lauenburg. While the territory of Saxe-Wittenberg changed drastically over the centuries, the Duchy of Lauenburg remained almost unchanged, until it lost its independence in 1689, when it was inherited by the Principality of Lüneburg. From 1815–1864 it was ruled in personal union by the Duke of Holstein, being simultaneously King of Denmark. In 1864 it fell to Prussia after the Second Schleswig War. For a short period Lauenburg was still an autonomous entity, but in 1876 it was incorporated as a district into the Prussian province of Schleswig-Holstein.
Lauenburg upon Elbe was the first capital and name-giving to the Duchy, before it moved in 1619 to Ratzeburg, which remained the capital also when the Duchy was downgraded to a district within Prussia. In 1937—by a territorial redeployment within the scope of the Greater Hamburg Act—the city of Geesthacht (formerly a part of Hamburg), some Mecklenburgian exclaves, including Ratzeburg's Cathedral immunity, and some Lübeckian exclaves within the district territory, were incorporated into the district.
In May 1945 British forces captured the district territory, which in September 1944 had been determined in the London Protocol to become part of the British Zone of Occupation. On November 13, 1945 the British general Colin Muir Barber and the Soviet general major signed the (also Gadebusch Agreement) in Gadebusch, redeploying some municipalities of the Duchy of Lauenburg District and neighbouring Mecklenburg, then part of the Soviet Zone of Occupation. Thus some eastern suburbs of Ratzeburg, such as Ziethen in Lauenburg, Mechow, Bäk and Römnitz became part of the district, while the Lauenburgian municipalities of Dechow, Groß and Klein Thurow (now component parts of Roggendorf) as well as Lassahn (now a component part of Zarrentin am Schaalsee) were ceded to Mecklenburg. The redeployment was accomplished on November 26, the respective occupational forces had until November 28 to withdraw to their respective new zonal territories.