What to Come after 1864 ?

After 1864, southern Jutland was a remote outskirts of the great German empire, and it weakened economic expansion and led to emigration and slow urbanization. Agriculture became more intensive and industrialization began, but only fully penetrated in Flensburg. The infrastructure was expanded with steamers, a modern road network and especially with railways.
In Denmark, after the New Year 1866, the government became aware that it was waging war between Prussia and Austria. The background was disagreement over Holsten, where after 1864 Austria had taken over the administration.
Following the advice of France, the Danish government offered its support to Prussia if Denmark recovered Schleswig north of Slien. However, Bismarck rejected pure, he did not need Danish support to defeat Austria.
On July 3, 1866, Prussia triumphed over Austria in the Battle of Sadova, and on August 23, with France as mediator, peace was made in Prague.
In Paragraph 5 of the Prague Peace, France was added:
“The inhabitants of the northern district of Schleswig must be relinquished to Denmark when, by a free vote, they express their desire to be united with Denmark.”
In the first decades after 1864, the Danish-minded Northern slayers (South Jews) protested vigorously against the annexation. This happened not least on the basis of Article 5 of the Prague Peace. This section was repealed in 1879.
The so-called optants and, not least, their children, the optant children, also caused problems between the Prussian administration and the South Jews.
The Protestants were the Danes who lived in the new Prussian North Schleswig between 1864 and 1870. They could stay and retain their Danish citizenship “if they were not in trouble”.
Otherwise, they could be deported to Denmark.
The optants could also choose to apply for German citizenship, which, however, was largely obstructed by the German local administration. It was worse with their children, the so-called opt-out children. They were born in Germany with Danish eyes and thus not citizens of Danish state. Conversely, with German eyes, they were born of Danish citizens and thus not German. In fact, they were stateless, and as time went on there were fewer people and more and more children.
The 1907 Optant Convention only partially solved this problem.
To prepare for a long-standing nationality struggle, the Danish movement organized from the 1880s into a number of large and small national associations. The connection to Denmark was strengthened in step with the intensification of the authorities’ attempts to compel Northern Schleswig with coercion.
In 1888, German became the only language of instruction in primary school, and the state at the same time initiated the acquisition of agricultural land to secure it in German hands.
During World War I, Southern Jutland men were deported to the Imperial German Army, and in German service lost about. 5000 shattered life while forced to fight on the German side. During the war, the region was drained of men to such an extent that Russian prisoners of war were deployed to help the farms.

The 1920 referendums.
The opportunity to get the duchies back was an option after 1918. But incorporating a Lauenborg with 90% German population and a Holstein with 75% German population would be unwise. By contrast, a vote in Schleswig where the northern one was predominantly Danish was a viable option. (Hitler then did not question this boundary, unlike other World War I borders.)
At the peace of Saint-Germain-en-Laye on September 10, 1919, in Article 85 of the Treaty, Austria waived its rights in the former Duchy of Schleswig. At the Grand Trianon peace on June 4, 1920, Hungary, in Article 69 of the Treaty, waives its rights in Schleswig.
In 1920, two referenda were conducted leading to the formation of the present frontier. On February 10, North Schleswig voted
a Danish majority of 75% of the votes for Denmark. On March 14, on the other hand, 80% of the voters in the Middle Swedes voting zone voted in favor of the area’s remaining in Germany. There were national minorities on both sides of the border.
Voting and reunification were obviously an event that called for all the national sentiment on both sides.