The Second Northern War (1655–1660), also known as the First Northern War or Little Northern War, was a significant conflict in Swedish history, showcasing Sweden's military might and expansion during the stormaktstiden (Age of Greatness). It was fought between Sweden and a coalition of adversaries including the Polish-Lithuanian Commonwealth, Russia, Brandenburg-Prussia, the Habsburg monarchy, and Denmark-Norway.
At the heart of the conflict was Charles X Gustav's ambition to expand Sweden’s power in the Baltic region. In 1655, Sweden invaded the Polish-Lithuanian Commonwealth, which was already weakened by internal conflicts and a Russian invasion. Sweden quickly occupied large portions of the Commonwealth in what became known as the Swedish Deluge. This expansion triggered a series of wars with Poland’s allies, including Russia and Denmark-Norway, as well as Brandenburg-Prussia, complicating Sweden’s position.
During the war, Sweden faced considerable resistance from Poland, where John II Casimir Vasa rallied support to reclaim his territories. As Sweden’s military overstretched, Denmark-Norway saw an opportunity to strike Sweden, leading to Sweden's dramatic winter crossing of the frozen straits to surprise and defeat Denmark in 1658. This resulted in the Treaty of Roskilde, where Denmark ceded significant territories, including Scania, Blekinge, and Halland, marking Sweden’s peak territorial gains in the war.
However, Sweden’s success was short-lived. Charles X Gustav’s decision to attack Denmark again in 1658 backfired. Sweden’s enemies regrouped, and by 1659, Sweden was on the defensive, protecting its holdings in Denmark and Swedish Pomerania. Following Charles X Gustav’s death in early 1660, Sweden, weakened by prolonged war, negotiated peace. The Treaty of Oliva (1660) with Poland, the Habsburgs, and Brandenburg restored peace and secured Sweden's control over Livonia. The Treaty of Copenhagen (1660) with Denmark forced Sweden to return Bornholm and Trøndelag but retained the key gains from the Treaty of Roskilde.
The Second Northern War solidified Sweden's dominance in the Baltic Sea region, marking the height of its territorial expansion, but it also highlighted the strains of overexpansion and foreshadowed the challenges Sweden would face in maintaining its empire.