During Rome's long sway over Western Europe, Roman provinces maintained trade routes with tribes in Denmark. One century after the Roman Empire collapsed upon itself in 410 AD, the first Danes are thought to have arrived, moving south into Denmark from Sweden.
Beginning in the middle of the 8th century, the Danes were known as Vikings. These rugged, sea-going adventurers (with their brothers from Norway and Sweden) raided, and then colonized many areas of Europe. They explored the North Atlantic Ocean, but their main focus was the British Isles and Western Europe, including Paris, France, (of all places) by traversing the Seine River.
In and around 965, Harald Bluetooth, son of the Viking King, Gorm the Old, united, then Christianized the Danes. In the early 11th century, Viking King, Canute the Great, rose to power and his forces conquered all of Denmark and Norway, and most of England.
In the early 14th century, the once-powerful realm of Danish Kings began to shrink, and in 1397, Denmark entered into a union (of sorts) with Norway and Sweden. This Kalmar Union of mostly self-serving dynasties dissolved in 1524, and war was on the horizon.
In the early 16th century, after Martin Luther nailed his (95 Theses) to the door of the Wittenberg Castle's Church, the Reformation began. Civil War and religious persecution swept Western Europe, and Denmark and Norway, now joined in union, were not immune.
In Denmark, the mayhem finally ended in 1536, and Denmark converted to Lutheranism. The Catholic Church was banished and beginning with King Christian IV in 1611, almost two centuries of war with Sweden followed.
Early military successes forced Sweden to pay ransom to Denmark, but no territorial changes occurred. Then, during the Thirty Year's War, the King and his forces suffered a devastating defeat and Jutland was occupied.
In 1642, adding insult to injury, Swedish armies invaded and Denmark was forced to surrender large areas of land including several provinces in Norway. King Frederick III declared war on Sweden in 1657, and this led to an almost total defeat of the Danish army.
Little Mermaid, Copenhagen Harbor