Why did the 1852 London Protocol matter in the German-Danish War?

The London Protocol of 1852 was a critical diplomatic agreement that ended the first war between Denmark and Prussia (1848–1850) by returning the duchies of Schleswig and Holstein to Danish control.12

Its significance in the German-Danish War of 1864 lies in the specific conditions it imposed on Denmark. Under the protocol, the Danish government pledged not to integrate Schleswig more closely into Denmark than it was tied to its sister duchy, Holstein.12

When the Danish government later violated this agreement by signing a new joint constitution for Denmark and Schleswig in 1863, it provided Prussia and Austria with a formal justification for military intervention. By positioning themselves as the upholders of the 1852 protocol, these powers were able to challenge Denmark’s actions, ultimately leading to the military conflict that resulted in Denmark ceding the duchies.12