At the start of the Great Northern War, Dücker became a subordinate to King Charles XII and served under him with distinction during the Polish and Russian campaigns.

[1][2] During the Invasion of Poland-Lithuania, Dücker accompanied the king in a rowboat during the crossing of Düna and served as an intelligence officer at the Battle of Kilszów.

[2] In 1704, he was placed in command of a 1,250 strong dragoon regiment composed of men Dücker had himself recruited from Danzig and the surrounding area the previous year.

[1][2] In 1706, whilst Charles' main army was advancing on Grodno, Russian and Polish forces had managed to cut off Swedish lines of communication with Livonia.

[2][4] Dücker was then promoted to lieutenant general and tasked with defending Swedish Pomerania from enemy incursions; However, despite initial successes, he was pushed back to Stralsund where he was then sieged down by a combined Russo-Danish army.

In October, after Stralsund had once again come under siege and Stettin had been captured, Dücker received orders from the Royal Council urging him to withdraw his troops back to Sweden.

[2] For the early part of 1714, Dücker then set about on building up Stralsund's defences; raising more troops and acquiring more funds, munitions and supplies.

[2] After Charles arrived at Stralsund following his exile in the Ottoman Empire, he assumed control of the city's defence and made Dücker his second-in-command.

[4] In 1715, despite the Swedes best efforts, they could prevent the Danes, Prussians and Russians from again besieging Stralsund, and Charles was soon convinced by Dücker and his other subordinates to return to Sweden.

[2][4] In April, Dücker took command of the army stationed in Strömstad and would serve under Charles during the invasion of Norway and the sieging of the Fredriksten fortress.

[2] According to a myth, shortly after the king's death, Dücker is said to have written a letter to Charles Frederick, Duke of Holstein, urging him to make a claim for the throne and to abolish the absolute monarchy, and that he would have full backing from the military.

[2][3] However, due to grief from the king's death, the duke became indecisive and failed to make a decision on the matter, to which Dücker is supposed to have said: Well, if he cannot be a man, then he must be a woman.

Whilst organising the defence of Sweden, he developed a close working relationship with Frederick, Prince of Hesse and would later support his ascension to the Swedish throne in 1720.

[8] During his time in the council, he advocated for an alliance with Great Britain and proposed a plan to send troops to Finland for a joint attack on Livonia, which had now come under Russian occupation.

[2] However, due to a lack of British involvement, and a growing distrust towards Britain in Sweden, Dücker was not able to carry out his plan to for attack on Livonia.

[2][4] In 1724, for his services in procuring an alliance between Sweden and Russia, he received a large sum of money by the Russian government and was given an estate in Livonia the following year.

[2][7] The two met the previous year after Dücker found her in a convent where her father, Prince Bogislaw Oginski, who was fighting against the Swedes, had hid her along with her sisters and cousins.