In the early 17th century, Denmark, under King Christian IV, became involved in the Thirty Years' War as part of its efforts to maintain influence in Northern Europe. The country faced rising tensions after Frederick V's deposition in 1623, which concerned Protestant leaders like John George of Saxony and George William of Brandenburg, who feared that the Holy Roman Emperor Ferdinand II aimed to reclaim former Catholic territories held by Protestants. As Duke of Holstein, Christian IV was also involved in the Lower Saxon Circle, a regional alliance, and was motivated to protect Danish economic interests, especially Baltic trade and tolls from the Øresund strait.
Tensions escalated when Ferdinand II allied with Albrecht von Wallenstein, tasking him with expanding Imperial control in northern Germany. In May 1625, the Lower Saxony kreis elected Christian IV as their military leader against the threat, although Saxony and Brandenburg resisted Danish involvement, viewing Denmark as a rival.
Denmark received financial support from the Dutch and English through the Treaty of The Hague in December 1625, allowing Christian IV to launch an ambitious military campaign. However, Danish forces suffered significant defeats, notably at the Battle of Lutter in August 1626. Many German allies were reluctant to fully support Denmark, and subsidies from the Dutch and English failed to materialize, weakening Christian's position. By the end of 1627, Wallenstein's forces occupied Jutland, Mecklenburg, and Pomerania, threatening Denmark's dominance in the region.
In 1628, as Wallenstein besieged the strategically important port of Stralsund, Swedish intervention, led by Gustavus Adolphus, prevented Denmark from further losses. Despite this assistance, Christian IV suffered another defeat at Wolgast and began peace negotiations with Wallenstein. Recognizing the growing threat of Swedish involvement, Wallenstein offered lenient terms, leading to the Treaty of Lübeck in June 1629. Under this treaty, Christian IV retained Schleswig and Holstein but had to abandon support for the German Protestants and cede Bremen and Verden, effectively ending Denmark's role as the dominant Nordic power.
The outcome of this conflict marked a significant turning point in Danish history. Christian IV's defeat diminished Denmark's influence in Northern Europe, signaling the end of its supremacy and paving the way for Sweden's rise as the leading power in the region. Additionally, the war drained Danish resources, and Christian IV's ambitions for territorial expansion were decisively curtailed, marking a shift in the balance of power in the Baltic.