The Nordic countries have had passport freedom since 1952, long before free movement within the EU.
But 2015 was a turning point in free movement within the Nordic region.
Sweden introduced ID checks on buses, trains and ferries to Sweden from Denmark, to limit immigration.
The Swedish decision prompted Norway to introduce border controls on all ferries from, among others, Sweden.
The vision - the most integrated region in 10 years
In August 2019, the Nordic prime ministers met in Reykjavik.
They presented a common vision - that the Nordic region will be the world's most sustainable and integrated region by 2030.
But at the end of the same year, Denmark introduced a temporary internal border control of immigrants from Sweden, to prevent Swedish gang criminals from entering the country.
- I do not think that the corona crisis should be seen as a reason for a lack of Nordic co-operation, but rather as a symptom that Nordic co-operation may not work as well as we have imagined, says Johan Strang, assistant professor of Nordic studies at the University of Helsinki.
"Consequences for how people organize their lives"
This spring, the Corona virus reached the Nordic countries, and in rounds Norway, Denmark, Iceland and Finland have had various types of border restrictions against, among others, Sweden, which has had its border open to neighboring countries.
- In the long run, I am a little worried that this occasional situation at the borders will have consequences for how people choose to organize their lives.
Will you apply for a job in the neighboring country, will you be able to buy a summer cottage in the neighboring country?
People will think about such things several times before they go on in the future, says Johan Strang.
More about this in Sunday's Agenda 21:15 SVT 2.