Standalone short: Bohus and Älvsborg, two rival fortresses that glared across the Göta älv for 300 years — until 1658 put them both on the same side and the border vanished. Leif + Margareta.
For three hundred years, two great fortresses glared at each other across this one river — one Swedish, one Norwegian. And then, almost overnight, they found themselves on the same side. This is how a border vanished.
Bohus was raised by Norway in 1308; Älvsborg held the Swedish side. The river between them was the frontier — and the Norwegians used it to charge a toll on every passing ship, slowly strangling Swedish trade. For centuries, this water was a wound that would not close.
Then the peace treaties did what armies never could. In 1645 Denmark gave up Halland to the south. And in 1658, at Roskilde, it gave up Bohuslän to the north — and with it, the fortress of Bohus. The two old rivals were suddenly Swedish, both of them.
And so Älvsborg became a strange thing: a border fortress with no border. The frontier it had been built to guard now ran far away. The most fought-over doorway in the north had quietly become the safe back-room of a kingdom.