In July 1361, Denmark’s King Valdemar IV (Valdemar Atterdag) took the island of Gotland, including the important trading town of Visby. This move threatened the commercial balance in the Baltic Sea by putting a key trade hub under Danish control. It set off a wider conflict between Denmark and leading Hanseatic cities.
On July 8, 1362, Danish forces defeated a Hanseatic fleet near Helsingborg in the Øresund (the strait linking the Baltic to the North Sea). The defeat weakened the Hanseatic military position and made it harder to pressure Denmark by sea. It also showed that control of narrow sea passages could decide the outcome of trade wars.
On November 19, 1367, Hanseatic cities and key partners in the Low Countries formed the Confederation of Cologne to coordinate war finance and naval action against Denmark. This was more than a merchant partnership: it was a wartime coalition designed to protect trade privileges and sea lanes. The alliance made large-scale, joint naval operations more feasible.
From April 4 to June 16, 1368, allied Hanseatic and Holstein forces besieged Copenhagen and destroyed Absalon’s Castle. The attack demonstrated that the coalition could strike Denmark’s key coastal center and disrupt royal power. It increased Danish pressure to negotiate by showing that the war was not confined to trade disputes.
On May 24, 1370, Denmark and the Hanseatic League signed the Treaty (Peace) of Stralsund, ending the Danish–Hanseatic War. The treaty is widely seen as a high point of Hanseatic political influence because it tied Danish royal decisions and key trade routes to Hanseatic approval. It also addressed control of ports and fortresses linked to the rich Scania herring markets.
A core result of the 1370 settlement was securing Hanseatic shipping access through “The Sound” (the Øresund), a chokepoint for entering and leaving the Baltic. By reducing Danish interference and toll pressure, the Hanse could move bulk goods—like timber, grain, and fish—more reliably. This improved the league’s ability to police and dominate long-distance Baltic trade after 1370.
The treaty granted the Hanseatic League control over key Scania fortresses connected to the herring fisheries and fairs, along with major shares of local revenues. Scania’s herring trade mattered because preserved fish was a staple food in much of medieval Europe, especially during fasting periods. With fortress control and commercial privileges, Hanseatic cities strengthened their role as the main organizers and profit-takers in this seasonal trade system.
Beyond trade, the 1370 peace gave the league a formal role in Danish royal politics by allowing it influence over the choice of king. This mattered because a friendly monarch could protect Hanseatic privileges, while a hostile one could close ports, raise tolls, or support rivals. The treaty therefore linked naval power and economic power to political outcomes.
King Valdemar IV died in 1375, creating a succession moment in which outside influence mattered. With the Treaty of Stralsund still shaping Denmark’s options, the Hanseatic League had leverage during the transition. In 1376, Olaf (son of Margaret and Haakon VI) was elected king of Denmark, aligning the crown with the broader Scandinavian power politics of the late 1300s.
In 1385, the special period of Hanseatic control tied to the Scania fortresses reached its planned end, and the fortresses were returned to Denmark. Even with the return, the 1370–1385 years had shown how a merchant-led coalition could use naval force and strategic chokepoints to win durable privileges. This marked a transition from direct control to negotiated influence.
In the 1390s, privateers known as the Victual Brothers operated in the Baltic and increasingly threatened merchant shipping. Their rise was tied to Scandinavian dynastic conflict, but the effect was commercial: raiding and blockades raised risk and cost for trade. The piracy problem tested whether Hanseatic “sea dominance” could be maintained without constant coalition warfare.
On June 17, 1397, the Kalmar Union brought Denmark, Norway, and Sweden under a single monarch, creating a larger political rival to Hanseatic influence in the region. This new union did not erase Hanseatic trading networks, but it strengthened Scandinavian state power over ports, tolls, and naval resources. As a result, the late-1300s peak created by the Treaty of Stralsund gave way to a more contested Baltic by around 1400.
Treaty of Stralsund (1370) and Hanseatic naval dominance in the Baltic (1370–1400)