“It is important to say that even though we want sharper controls as an assurance against crime and human smuggling," Jensen was reported as saying, "it is also important that it occurs within the framework of our shared playing rules, in other words within Schengen."
As of 2015, all but six E.U. nations have signed up, with the exception of Britain and Ireland, who opted out, and four other newer E.U. member states who are legally obligated to join. A number of non-E.U. states, such as Switzerland and Iceland, are also members.
Outside of the euro, the Schengen area might be one of the most remarkable features of European integration for the average citizen, making travel between the nations astoundingly easy. It opened up the ability to work and live in another European state without being subject to border checks, a huge change for millions of people.
For a continent where countries were once divided quite literally by walls, the whole thing took on a symbolic importance. "The creation of the Schengen area is one of the greatest achievements of the E.U., and it is irreversible," Dimitris Avramopoulos, the European Commissioner for Migration, Home Affairs and Citizenship, said this month on the area's 30th birthday.
However, for all the benefits the Schengen area brings, there are also substantial difficulties. Immigration is a particular concern: In the past, Italy has been accused of allowing the many migrants and asylum seekers who showed up on its doorstep to escape across Europe's uncontrolled borders. The Hungarian government has become so incensed with the migrants who sneak into the country, often with the hope of going on further into the Schengen area, that's its proposed building a giant wall on its border with Serbia (a non-E.U. member).
In addition, the open borders created drug-trafficking problems. Some European countries with strict gun control laws have even complained that weapons can be smuggled across the border from other nations with relative ease.
The Schengen area does allow for some temporary reinstatement of border controls in special circumstances. Article 2.2 of the treaty allows for a brief reinstatement for "public policy or national security" reasons. France reinstalled border controls after the bomb attacks in London in 2005, for example, and Germany put some border controls in place during the 2006 World Cup.
However, other attempts to reinstate border controls have been met with outrage from E.U. officials.
This isn't the first time that Denmark has pushed back against the laws: For a brief period in 2011, a center-right government reinstalled the controls on its borders with Germany and Sweden, bowing to pressure from the populist Danish People's Party (DF). The E.U. was outraged, but the decision was reversed a few months later by the newly elected Social Democrats.
Now, following an election earlier this month that saw the Social Democrats forced to form a minority government and the DF become the second largest party in the country, border controls will be reinstated again.
It's unclear how Denmark can reinstate border controls while remaining faithful to the ideals of the Schengen area. Foreign ministry spokesperson Lars Peters told AFP that there would not be a "border barrier" or "checks in the border areas," and some analysts have said the move may be more symbolic than practical. The E.U. Commission seems to be cautious about condemning the plan so far – it has said it will not comment on the Danish plan until more details become available.
Even so, the symbolic power of such a move might be powerful. The DF, for example, is just one of a number of anti-E.U. populist parties that have surged in popularity over the past few years. Like these other parties, it seeks to leave the E.U. or at the very least significantly restructure Denmark's relationship to it.
And Denmark isn't the only country challenging the Schengen area this week: On Thursday, a French court ruled that the arrest of migrants on its border with Italy "exceed the legal framework [of the Schengen area], be it by their magnitude, frequency or implementation."