1. Denmark's Flag Is Very Old, and Its Origin Story Is a Legend
Denmark's flag is called the Dannebrog, a name that probably means "the cloth of the Danes." According to a legend, a red banner with a white cross fell from the sky on June 15, 1219, during a battle in what is now Estonia, and it turned the fight in the Danish army's favor [1].
That legend was first recorded by historians in the early 16th century, about three hundred years after the battle. Denmark's official portal says plainly that the falling flag has no basis in reality, though the crusade itself did happen [1]. What is on firmer ground is the flag's later history. A red flag with a white cross was used by many crusaders and was not tied to Denmark at first. It became a Danish flag around the middle of the 14th century, which makes it one of the world's oldest national flags in continuous use [1]. (Different lists measure "oldest" differently, so "one of the oldest" is the safe way to say it.)
So by the time the other Nordic flags were designed, Denmark already had a red-and-white cross flag that people in the region had seen for generations. Which neighbors used it, copied it, or answered it?
2. Why the Cross Spread: Used Every Day, Copied as a Model, or Chosen as a Sign of Belonging
The sources point to three different routes. No single reason covers all five countries.
Norway used the Danish flag for years
Norway was part of the Danish-Norwegian state until 1814, and during that time it had no flag of its own. It simply used the Dannebrog [2]. A cross flag was therefore the one Norwegians were used to seeing. (That it felt familiar is a reasonable guess of mine, not something the Norwegian parliament says.)
Sweden most likely took Denmark as its model
The Swedish National Archives says that the history of Sweden's flag goes back to the middle of the 1500s, and that its model was "surely" taken from Denmark, which had long had the red-and-white cross flag as its symbol [3].
Finland and Iceland use the cross as a link to the North
For Finland, a Danish reference work on flags says the cross is meant to show the country's tie to the other Nordic countries [5]. For Iceland, the same reference work says the cross stands for the connection to Scandinavia [6]. These two are closer to "we are part of this group" than to "we copied that flag."
The sources do not give a detailed debate for why Finland or Iceland picked a cross, so it would be going too far to call the Danish flag the ancestor of all of them. If the cross is shared, why do the colors differ so much? Norway's story is the best documented, so let us start there.
3. Why Norway's Parliament Voted for a New Flag in 1821
In 1814 Norway was forced into a union with Sweden. That left a practical problem. The Danish flag no longer fit, and Norwegian trading ships could use only the union flag: the Swedish flag with a red-and-white diagonal cross (a St. Andrew's Cross) in one corner [2].
In autumn 1820 a merchant and shipowner from Stavanger, Gabriel Schanche Kielland, asked the Storting (Norway's parliament) to choose a Norwegian trade flag, and many people along the coast backed him. In 1821 a member of parliament from Bergen, Frederik Meltzer, argued for a flag that would be a clear break from all other national flags. It should have a cross and be red, white and blue, which the Storting describes as the colors of liberty, used on the flags of the French and American revolutions [2].
Notice that a cross was part of the proposal from the start, even though Meltzer wanted something unlike other flags. The Storting's page does not say why he chose a cross, so that question stays open.
When parliament met again on May 4, 1821, Meltzer's design received the most votes. The Swedish-Norwegian king, Carl Johan, did not like the idea that parliament could decide a flag, which he saw as his own right. He refused to approve a law, but he issued a royal decree making the design the Norwegian trade flag, and the Storting let that stand [2]. The Flag Act of 1898 then removed the union emblem from the Norwegian flag, giving the flag Norway uses today [2].
So Norway kept the cross but changed the colors to say something about itself. Did the other countries do the same?
4. Where the Colors of the Swedish, Finnish and Icelandic Flags Come From
Sweden: the blue and yellow come from the national coat of arms, which shows three golden crowns on a blue field. The arms are divided into four blue fields by a cross, and the Swedish National Archives connects the two ideas [3]. Putting these together, a good reading (mine, not the archive's) is that Sweden kept the Danish-style cross shape but filled it with the colors of its own arms.
Finland: the flag, a blue cross on white, was made official by a law of May 29, 1918, less than six months after independence. The poet Zacharias Topelius had described the colors in a poem first published in 1863 as "the blue of our lakes and the white snow of our winters" [4]. The first sketches were drawn by the artists Eero Snellman and Bruno Tuukkanen [5].
Iceland: the flag has been official since 1915. Blue and white are Iceland's traditional colors, and red was added [6].
Put side by side, the pattern is clear. The cross is shared through habit, example or a wish to belong. The colors were chosen separately: liberty for Norway, a coat of arms for Sweden, lakes and snow for Finland.
5. How the Danish Flag Went from a Royal Banner to Something Hung on Houses
The flag at the center of the story had an unexpected history inside Denmark. For centuries it was mainly a royal flag. In 1834 the king even banned ordinary citizens from using it. Yet when Danish soldiers came home from winning the First Schleswig War (1848-1850), they were met by red-and-white flags hanging from private houses across the country, and the ban was formally lifted a few years later [1].
Today there is no law governing the most common version of the flag in Denmark, only guidelines, and people use it for birthdays, funerals and many things in between. Since 1913, June 15 has been celebrated as Valdemar's Day, after the king of the legend [1]. A historian quoted by the Danish portal says that using the flag in so many ways is what makes it a uniting symbol [1].
Norway's vote, Finland's poem and Denmark's backyard flags point the same way: a flag becomes part of a country when the people, not only the king, start to care about it. (That link between the three stories is my own observation. None of the sources states it as a single trend.)
6. Try It Yourself: Compare the Five Flags and Check the Sources
Find a flag book or atlas at home or in a library (or compare two trustworthy flag websites), and look at the flags of Denmark, Norway, Sweden, Finland and Iceland. Sketch each one on paper and note three things: where the cross sits, how many colors it has, and what color the background is. Then ask whether the three routes from this article (everyday use, a model, belonging) seem to fit each flag. If one does not fit, you have a new question to investigate.
You can also read the original pages listed below. The Norwegian parliament's page tells the 1821 story in English, and the Swedish National Archives page is short enough to read in Swedish with a translation tool. Two questions are still open: exactly why Finland and Iceland chose crosses, and how historians decide which flag is "the oldest."