Denmark once held the Baltic straits alone: no ship passed through the Øresund strait without its permission. The Swedish Empire that eventually brought Denmark to its knees grew out of a country Copenhagen had treated as a province, and their battle for the Baltic lasted more than two hundred years. It began with the Stockholm Bloodbath and Gustav Vasa's uprising, which broke the Kalmar Union. Gustavus Adolphus rebuilt the army and the state. In midwinter Charles X Gustav marched his army across the frozen Belts toward Copenhagen, and at Roskilde Denmark gave up Scania, its richest province, and soon its great-power status as well.

Sweden's victory proved fragile: an empire from Pomerania to Karelia rested on barely a million and a half people, Finland included, and the balance held only under Charles XI, who even defended Scania at Lund. His son Charles XII followed early victories with six years bogged down in Poland, lost at Poltava, and was killed at Fredriksten, after which the Treaty of Nystad took Livonia, Estonia, and Ingria from Sweden. Neither crown ever became master of the sea, and that unfinished rivalry is what shaped the borders of modern Scandinavia.

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