When French armies besiege Starlsund in 1807, King Gustav IV Adolf is eager to gain some military glory and sails south with reinforcements earlier than in OTL. Once a stray artillery round kills him, poor Crown Prince Gustav and his regency council are wise enough to sue for peace. With Sweden back in fold, there's no need of the Finnish War and the eastern border of Sweden remains in River Kymi. Later on the council captures Norway from Denmark.
Once Crown Prince comes of an age and begins his reign as Gustav V, the country enjoys moderate peace, but the rising tide of nationalism in Norway and to lesser extent in Finn-inhabitated eastern parts of Sweden starts to cause trouble. Later on Swedish Empire, still harbouring ideas of revanche against Russia, joins to Central Powers. The northern front turns into a bloody trench warfare in the forests of eastern Karelia, and instead of a swift offensive to Petrograd, the Swedes are only able to reach Vyborg after years of bitter combat and sizeable German support.
The Treaty of Brest-Litovsk seemingly satisfies the territorial ambitions of Swedish Empire by ceding away territory all the way to river Svir in the east, but by then it's too late for the Central Powers. As the Western Front starts to crumble, the Finnish regiments who've been throughly demoralized by Finnish-speaking Bolshevik agitators from Karelia and Ingria start to form soldier soviets and first ignore and then outright kill their Swedish officers.
Meanwhile unrest in Norway gets out of hand as well due the food shortages and soaring inflation. As deserting Finnish soldiers start to form Red Guards in Finnish provinces of eastern Sweden around Vyborg, Vildmanstrand and Lahtis. The rebellion soon starts to spread westwards. As the rebels gain support of the large group of tenant farmers and industrial workers, atrocities against Swedish-speaking priests and nobles start to occur through the eastern half of the country. They are soon revenged by members of
Frikåren, volunteer paramilitary units that are formed from veterans eager to retain the cohesion of the dying empire.
As the Swedish Empire sues for peace, victorious Allies are willing to give the Finnish nationalists new borders all the way to Tornio River. Frikåren forces, led by a respected wartime commander Carl August Ehrensvärd, refuse to acknowledge the treaty and continue their bitter civil war in eastern Sweden to secure more favourable borders. Later on League of Nations enforces the bitter peace terms by referendums that are widely seen as manipulated to favour the Finnish territorial claims, and half of historical Sweden is ceded to Republic of Finland alongside the vitally important Åland Archipelago. As the left-leaning Republic of Finland soon signs treaty of friendship and cooperation with Soviet Union followed by a coup that turns the country to Peoples Republic and a virtual puppet of Moscow, Swedish hopes for a revanche are seemingly fading fast.
As the new Peoples Republic of Finland officially blames the large Swedish minority for centuries of oppression against Finnish peasantry, they also start a wide-spread Finnification campaign. As the new rump Swedish state receives a steady flow of refugees from former Ostland, the bitterness towards Entente peace terms and the hated treaty of Versailles soon allows radical nationalist leaders to take over, while Social Democracy is widely discredited by it's association with the terror of Finnish Red Guards. Racial hatred towards the Finns and other national minorities is turned into political weapon, and later on it becomes an official ideology in the idea of
folkhemmet, an organic society of all racially pure Swedes.
The pan-Scandinavian expansionism of Swedish fascism is part of the forces that push Europe to a new world war. Allied with the revisionist military junta of Deutschvölkische Freiheitspartei, Swedish forces participate to occupation of Denmark and Norway and finally to the long-awaited invasion of Finland and Soviet Union. Decades of propaganda about the attrocities "Turano-Mongoloid Finns and their Judeo-Bolshevik masters" had alledgedly brought upon the oppressed Swedes in Finland led to grim results, as the advancing Swedish troops viewed the locals as subhumans and treated them accordingly, especially after the Finnish guerrillas started a bitter partisan war. By the time the Red Army supported by formations of Suomen Kansanarmeija reached Uleåborg, and the retreating Swedish forces withdrew towards northwest, it was clear that Finland had been throughly devastated by scorched earth tactics and years of anti-partisan warfare. Postwar census found out that only Belarussian SSR had suffered more losses of her pre-war civilian population.
Brr, it's a really grim scenario now when I really think of it
