In support of his maternal uncle Christian IV, King
Charles I of England, Ireland, and Scotland had sent English and Scottish
mercenaries, who stood in the western Schleswig marshes. In June 1629 Ulrik had the task of ensuring their repatriation over the
North Sea. He then travelled via
Glückstadt again to the Dutch Republic, fighting under
Stadtholder Frederick Henry, Prince of Orange in his
Siege of 's-Hertogenbosch until its surrender on 14 September 1629. Thereafter Ulrik returned to Denmark. In April 1630 he accompanied his father in his campaign against
Hamburg, where Ulrik got into mortal danger. In the same time Christian IV tried a rapprochement with
Emperor Ferdinand II and Wallenstein in order to regain Schwerin and Verden for his deposed sons. To this end Ulrik participated in the
Regensburg Diet of the
prince-electors in July to November 1630 (), where he conferred with
Leopold V, Archduke of Austria, brother of Ferdinand II, and Wallenstein, however, all in vain. The prince-electors, fearing too strong a position for the emperor, had enforced Wallenstein's dismissal and suspended the
Restitution edict, which enabled the Catholic emperor to restitute church property and possessions, held by defeated Lutherans, to the Catholic Church. Ulrik travelled to England via the Dutch Republic in order to visit his cousin King Charles I, persuading him to pay Ulrik an annual pension. Ulrik spent the winter of 1630/31 in Denmark, before leaving in spring again to fight for the Protestant rulers of
Brandenburg and
Saxony. Meanwhile, Swedish Lutheran troops had conquered the Catholic-occupied Prince-Bishopric of Schwerin, so that Ulrik hoped to regain his realm from Gustavus Adolphus. To this end Ulrik even considered the marriage with Princess
Christina of Sweden. But regardless of this endeavor and new negotiations from Christian IV's side with the princes, the emperor and Wallenstein did not accept Ulrik's restitution. Tired of travelling, Ulrik got his father's consent to enter into Saxon military service. In February 1632 he left for
John George I, Elector of Saxony. He disliked
Dresden court life, where people were more concerned about a good living than the ongoing war. So Ulrik was delighted once he set out for a campaign as a colonel in the Saxon army in March 1632. He advanced to the rank of general of the electoral Saxon artillery. In Denmark Ulrik recruited a
cuirassier company under his command, and – in summer – he joined with it the electoral Saxon army under
Hans Georg von Arnim-Boitzenburg for
Silesia. He probably participated in the conquest of
Gross-Glogau and stayed in
Neisse later the year. In a
Jesuit College there he found
Tycho Brahe's celestial globe, which he sent home to Denmark as
war booty. After a quiet and peaceful winter in 1632/1633 fighting resumed in January, and Ulrik had the opportunity to excel. At the same time the plan for Ulrik's marriage to Christina again surfaced, but was rejected by
Axel Oxenstierna. While Ulrik's own interest was still more concerned about the Prince-Bishopric of Schwerin. In May 1633 Wallenstein had been reappointed in charge of the imperial army. His attempts to negotiate with the Protestant enemy regularly gave rise to armistices, and during one of them Ulrik met Wallenstein. During the war efforts, interrupting the negotiations, Ulrik excelled again, inflicting the imperial
Croatian riders a significant defeat. New peace talks began between the two sides, and Ulrik participated in them. On 11 August 1633, during one of these meetings in
Schweidnitz, he was fatally wounded by an unexpectedly treacherous shot from an imperial horseman, and died the following night. His body was first taken to
Liegnitz, thence to Dresden, where it stayed until spring 1634. After a funeral, it was then brought to
Copenhagen, where it was set up in the
Church of Our Lady, until his mortal remains found their final rest in
Christian IV's chapel in
Roskilde Cathedral in 1642. ==Other occupations==