The two hundred years of peace that characterise Sweden's modern history began with a disastrous war against Russia, 1808–1809. Part of the French Revolutionary and Napoleonic Wars, it is probably little known on the Continent, yet it had lasting consequences. Sweden lost Finland, accounting for a third of Sweden's territory and a quarter of her population. From being a maritime empire in the Baltic Sea, Sweden contracted into the territorial state of its present shape. Stockholm, the capital at the heart of the old Swedish Baltic Sea empire ended up on the easternmost corner of the new territory that took shape in 1809, just hours in distance from the Russian-controlled Åland islands – a part of then Russian Finland. This was perhaps the biggest geopolitical shift in Sweden's history.

The disastrous war entailed a political revolution that removed King Gustaf IV Adolph; within a year he was replaced by the French General Jean Bernadotte, the future King Karl XIV Johan. Sweden became a constitutional monarchy. In the final years of the French Revolutionary and Napoleonic Wars Sweden also accepted Prussia's incorporation of Swedish Pomerania, a remnant of the seventeenth-century great-power period. In exchange, in the European settlement of states 1814–15, Norway was transferred to Sweden from Denmark. While Sweden under Bernadotte took an active part in the campaign against Napoleon, Denmark stayed a loyal ally to France until the end.