Treaty of Elbing The Treaty of Elbing was signed between the Dutch Republic and the Swedish Empire on 1 September ( OS ) / 11 September 1656 , during the Second Northern War , in Swedish - held Elbing ( Elbląg ) . It served to protect Dutch interests in the Baltic Sea , ended the Dutch intervention in the Swedish siege of Danzig , and renewed a fragile peace between the Dutch Republic and Sweden . Within the former there was opposition to the treaty demanding elucidations , which were agreed upon only on 29 November ( OS ) / 9 December 1659 in the Convention of Helsingör . Earlier in 1659 , in the Concert of The Hague , England , France , and the Dutch Republic had agreed to include the treaty of Elbing in their common agenda regarding the Second Northern War . Background . During the Second Northern War , Charles X Gustav of Sweden aimed at establishing a Swedish dominion in Royal Prussia , part of the Polish – Lithuanian Commonwealth , and subdued most of the province in a campaign of December 1655 . The Dutch Republic heavily relied on grain imports from the Baltic Sea region , much of it was bought from Royal Prussia ' s chief city Danzig ( Gdansk ) . When Swedish forces began raising a siege of Danzig in early January 1656 , the key figures of contemporary Dutch politics , Johan de Witt and Coenraad van Beuningen , saw the Dutch trade in the Baltic endangered and feared an alliance of a strengthened Sweden with their rival Maritime Power , England . Van Beuningen , at that time residing in Copenhagen at the court of Sweden ' s adversary and Dutch ally Frederick III of Denmark , proposed declaring war on Sweden . De Witt favoured a more moderate approach , fearing follow - up declarations of war on the Dutch Republic by England and France . De Witt instead proposed aiding Danzig directly by dispatching a relief force , and in July gained consent for sending in a fleet and a landing force by the States - General of the Netherlands due to diplomatic efforts of Holland ' s diplomats in The Hague . In late July , forty - two Dutch and nine Danish vessels arrived in Danzig , carrying 10 , 000 soldiers and 2 , 000 guns . The army made landfall and a Dutch commander took over Danzig ' s defense . Christer Bonde , Sweden ' s ambassador in London , alerted Lord Protector Oliver Cromwell who sent letters to Sweden and the Dutch Republic urging for a peaceful settlement . Terms . The treaty was concluded on 11 September 1656 , which was 1 September on the Julian calendar then used in the Baltic area . Two earlier Dutch - Swedish trade agreements of 1640 and 1645 were confirmed . Sweden assured Dutch merchants free passage to Danzig as well as free trade and navigation in the Baltic Sea . The port dues in trade cities like Danzig and Pillau ( now Baltiysk ) were not to be raised . Sweden further promised to respect Danzig ' s neutrality in the Second Northern War , and granted the Dutch Republic the status of a " most favoured nation . " Sweden and the Dutch Republic assured mutual friendship . Implementation . The treaty met with opposition by several Dutch towns and Denmark . The Dutch opposition against the treaty was led by Van Beuningen , who refused to ratify it and looked forward to confront Sweden on the Danish side , while De Witt stood by the treaty and his strategy to aid Denmark and be concilatory with Sweden at the same time . De Witt proposed ' elucidation ' of the treaty rather than abandoning it , and the States - General accepted . In 1657 , Frederick III of Denmark declared war on Sweden , but his invasion was repelled by Charles X Gustav who subsequently took all of Denmark except for the capital city Copenhagen . With the consent of De Witt , who thought that a balance of the Danish and Swedish powers would serve Dutch interests best , and despite the protest of part of the Dutch elites , Dutch auxiliaries were deployed to relieve the Swedish siege of Copenhagen . De Witt realized that the intervention on the Danish side bore the risk of provoking England , who was then allied to France . Accordingly , the relief forces were advised to avoid any encounters with the English navy , and De Witt furthered negotiations with English and French diplomats to agree on a common stance regarding the Second Northern War . An agreement was reached in The Hague , drafted by English envoy Downing , which encompassed the treaty of Elbing along with its elucidations . The agreement was endangered when after the abdication of Richard Cromwell , Downing was instructed to remove the passages regarding Elbing , yet in May 1659 the new English government approved the original draft . The agreement , ratified on 21 May , became known as Concert of The Hague . On 29 November ( OS ) / 9 December 1659 , all Dutch parties and Sweden had settled for the elucidations of the treaty of Elbing , which was then accepted by all parties in the Convention of Helsingör , concluded in Helsingør ( Helsingör , Elsinore ) .