European powers signed the London Protocol to preserve the Danish monarchy’s overall territorial integrity while addressing succession in the duchies of Schleswig, Holstein, and Lauenburg. It aimed to prevent a power shift in the Baltic region, especially control of the strategic port area around Kiel. The agreement became a key reference point when Denmark later tried to tighten its ties with Schleswig.
King Christian IX signed Denmark’s “November Constitution,” creating a closer constitutional link between Denmark and Schleswig while separating Holstein. Prussia and Austria argued this violated earlier international agreements (including the 1852 London Protocol). The move escalated a legal and political dispute into an immediate international crisis.
The German Confederation responded by authorizing a “federal execution” (a legal enforcement action) against Holstein and Lauenburg. Saxon and Hanoverian troops entered Holstein to pressure Denmark and assert the Confederation’s position. This step helped set the stage for Prussia and Austria to take the lead in the conflict.
Prussian and Austrian forces crossed into Schleswig, starting the Second Schleswig War. The invasion followed Denmark’s refusal to withdraw the November Constitution after Austro-Prussian demands. The war shifted the duchies question from diplomacy to armed conflict, with Prussia and Austria acting together.
As Danish forces withdrew from the Dannevirke position, they fought a rearguard battle at Sankelmark (also called Oeversee). The clash did not decide the war, but it showed how quickly Denmark was being pushed northward. It also became a lasting memorial point in later Danish and German remembrance.
Prussia captured the Danish fortified line at Dybbøl after a siege and a major assault. The defeat severely weakened Denmark’s position both militarily and in the ongoing peace talks. Dybbøl became the best-known battle of the war and a symbol of Denmark’s loss of the duchies.
A European conference convened in London to end the fighting and settle Schleswig’s future. Great-power diplomacy tried to produce a compromise (including proposals to partition Schleswig), but the talks were weakened by events on the battlefield. The conference ultimately failed to reach an agreement.
Negotiations ended in London with no final settlement, and the truce lapsed. With diplomacy stalled, Prussia and Austria resumed operations to force Denmark to accept harsher terms. The failure marked a turning point toward a military-imposed outcome rather than a negotiated compromise.
Prussian forces seized the island of Als in a night attack, defeating Danish troops stationed there. This was the last major engagement of the war and removed Denmark’s remaining strong defensive position near the contested area. After Als, Denmark’s ability to continue fighting effectively collapsed.
Denmark signed the Treaty of Vienna with Prussia and Austria, ending the war. Denmark ceded Schleswig, Holstein, and Lauenburg to the two victors, who agreed to hold the territories jointly (a shared sovereignty arrangement often described as a condominium). The treaty shifted the conflict from Denmark versus Germany to a dispute between Prussia and Austria over control.
Prussia and Austria renegotiated their joint rule of the “Elbe Duchies” to reduce friction. The Gastein Convention assigned Prussia to administer Schleswig and Austria to administer Holstein, while Prussia also gained rights connected to Lauenburg. The arrangement postponed conflict briefly but did not resolve the underlying rivalry.
Both states ratified the Gastein Convention, putting the new administrative split into effect. In practice, the division created a fault line: Prussia controlled the northern duchy (Schleswig) while Austria held Holstein in the south. This set up the next phase of the unification-era struggle, as disputes over Holstein soon became a trigger for war between the former allies.
Second Schleswig War and the Prussian annexation of Schleswig-Holstein (1864–1865)