Before 1864 Schleswig was a fiefdom of Denmark, while Holstein was a fief of the Holy Roman Empire (until 1806) and a member of the German Confederation (after 1815).

Prime minister Lars Løkke Rasmussen cited fear of accumulation of illegal migrants in Copenhagen as one of the reasons for this decision.

[5] In January 2019, the Danish government began constructing a fence along the border to keep wild boar, which can carry African swine fever virus, from crossing into Denmark.

The 1.5 metres (4.9 ft) high, nearly 70 kilometres (43 mi) long fence—spanning the entire land border—was completed in December 2019 at an estimated cost of 30.4 million Danish kroner.

[9] After completion there was a decision to raise it by adding wires over it, because animals like deer have been killed after being injured because of jumping over the fence.

'bird flight line' in German), which included a train ferry over the countries' maritime border (between Rødby and Puttgarden), closed in December 2019.