Return to service
Once released, Dücker then travelled to the town of Rastenburg before making his way to the city of Stralsund. On 31 January 1710, he was appointed to major general. In late February, Dücker met up with General Magnus Stenbock at his headquarters in Scania and would go on to serve under him at the Battle of Helsingborg. For his actions at the battle, he was given command of the Västergötland Cavalry Regiment. 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. After Stenbock relieved the city, he then placed Dücker in charge of his army's vanguard. During the Battle of Gadebusch, he was severely wounded in the neck by a musket ball, and had to be taken to Lübeck to have it removed.
Following his recovery, in February 1713, Dücker attempted to try and re-join Stenbock's army besieged in Tönning. He planned to sneak into the town by disguising himself, but this was abandoned. Dücker then returned to Stralsund where he resumed command of the city as well as all Swedish forces in Pomerania. In March, he was appointed general of the cavalry. 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. Dücker, however, refused these orders, saying that he wouldn't leave unless he was commanded to the king. 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. In May 1714, he managed to successfully incorporate two regiments from Holstein-Gottorp under Swedish command.
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. 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. Dücker volunteered to remain in Stralsund and continued defending the city until December, when he was finally forced to surrender.
After being taken prisoner, Dücker was allowed to go to Hamburg on a word of honour so he could nurse his wounds. He stayed in Hamburg from 1716 to 1717 before travelling to London and then to Gothenburg, arriving there in January 1718. 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. Following Charles' death, Dücker took part in the decision to lift the siege and return to Sweden. Afterwards, Dücker, along with a few other generals, made a declaration that they would not swear an oath of homage until the royal election was held. Following the ascension of Ulrika Eleonora, they then swore the oath.
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. 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. And now it is as good as too late. We lack no regent".
