First Schleswig War The First Schleswig War ( ) or Three Years ' War ( ) was the first round of military conflict in southern Denmark and northern Germany rooted in the Schleswig - Holstein Question , contesting the issue of who should control the Duchies of Schleswig and Holstein . The war , which lasted from 1848 – 1851 , also involved troops from Prussia and Sweden . Ultimately , the war resulted in a Danish victory . A second conflict , the Second Schleswig War , erupted in 1864 . Background . At the beginning of 1848 , Denmark contained the Duchy of Schleswig , Duchy of Holstein and Saxe - Lauenburg , where the majority of the ethnic Germans in Denmark lived . Germans made up a third of the country ' s population , and the three duchies were behind a half of Denmark ' s economic power . The Napoleonic Wars , which ended in 1815 , had increased Danish and German nationalism . Pan - German ideology had become highly influential in the decades prior to the war outbreak and writers such as Jacob Grimm argued that the entire Peninsula of Jutland had been populated by Germans in before the arrival of the Danes and that therefore it could justifiably be reclaimed by Germany . These claims were countered in pamphlets by Jens Jacob Asmussen Worsaae , an archaeologist who had excavated parts of Danevirke , who argued that there was no way of knowing the language of the earliest inhabitants of Danish territory , that Germans had more solid historical claims to large parts of France and England , and that Slavs by the same reasoning could annex parts of Eastern Germany . The conflicting aims of Danish and German nationalists was a cause behind the First Schleswig War . Danish nationalists believed that Schleswig , but not Holstein , should be a part of Denmark , as Schleswig contained a large number of Danes , whilst Holstein did not . German nationalists believed that Schleswig , Holstein , and Lauenburg should remain united , and their belief that Schleswig and Holstein should not be separated led to the two duchies being referred to as Schleswig - Holstein . Schleswig was a particular source of contention , as it contained a large number of Danes , Germans and North Frisians . Another cause of the war was the illegal introduction of a royal law in the duchies . When King Christian VIII of Denmark died in January 1848 , and seeing that his only legitimate son , the future Frederick VII , was apparently unable to beget heirs the duchies could have gone under the rule of the House of Oldenburg , which might have resulted in a division of Denmark . As a result , a royal law was decreed in the duchies that would allow a female relative of Christian VIII to assume control . The implementation of this law was illegal . Trigger . On March 21 , 1848 , a crowd of Danish citizens in Copenhagen demonstrated for the creation of a liberal constitution for Denmark . Local government officials and other important people in Kiel , a city in Schleswig - Holstein and a settlement from which the German nationalist party operated , claimed to not not know fully what was going on in Copenhagen , and proclaimed a provisional government on March 24 . They did this because they thought the King had fallen under the control of the revolutionaries who wanted to separate Schleswig from Holstein . The declaration was considered an act of rebellion by the central Danish government . Schleswig - Holsteinian Prince Frederik of Noer took the 5th " Lauenburger " Rifle Corps ( Jägerkorps ) and some students of Kiel university to take over the fortress of Rendsburg in Schleswig - Holstein . The fortress contained the main armoury of the duchies , and the 14th , 15th , and 16th Infantry Battalions , the 2nd Regiment of Artillery , as well as some military engineer s . When Noer ' s force arrived , they found that the gates to the fortress had been left open for an unknown reason , and promptly walked in , surprising the would - be defenders . After delivering a speech to the defenders , the prince secured the allegiance of the battalions and regiment of artillery to the provisional government . Danish officers who had been serving in the defense of the fortress were allowed to leave for Denmark , but on the assurance that they did not fight against Schleswig - Holstein in the coming war . Course of the war . 1848 . Wishing to defeat Denmark before Prussian , Austrian , and German troops arrived to support them , 7 , 000 Schleswig - Holsteinish soldiers under General Krohn occupied Flensborg on March 31 . Over 7 , 000 Danish soldiers landed east of the city , and Krohn , fearing he would be surrounded , ordered his forces to withdraw . The Danes were able to reach the Schleswig - Holsteiners before they were able to retreat , and the subsequent Battle of Bov on April 9 was a Danish victory . At the battle , the Prince of Noer , senior commander of the Schleswig - Holsteinish forces , did not arrive until two hours after fighting had started , and the Schleswig - Holsteiners were more prepared for the withdrawal they had intended to make before they were attacked than for an engagement . By entering the war on behalf of Schleswig - Holstein , the Prussians were acting on behalf of a democratic uprising . This was out of the norm for the Prussians . This was the only time that Prussia carried out assistance for a revolutionary movement . Indeed , concurrently with their participation in the First Schleswig - Holstein War , Prussia , was occupying the Polish city of Posen to put down a revolutionary uprising there , and was fighting in Italy and Hungary to put down revolutionary uprisings . The Germans had embarked on this course of participation in the Schleswig - Holstein War alone , without the European powers . The other European powers were united in opposing any dismemberment of Denmark , even Austria refusing to assist in enforcing the German view . Swedish troops landed to assist the Danes ; Tsar Nicholas I of Russia , speaking with authority as head of the senior Gottorp line , pointed out to King Frederick William IV of Prussia the risks of a collision . Great Britain , though the Danes had rejected her mediation , threatened to send her fleet to assist in preserving the status quo . The fact that Prussia had entered the war on behalf of the revolutionary forces in Schleswig - Holstein , created a great number of ironies . The newly elected Frankfurt Diet tended to support the incursion into the Schleswig - Holstein War while King Frederick William did not . Indeed , Friedrich William ordered Friedrich von Wrangel , commanding the army of the German Confederation , to withdraw his troops from the duchies ; but the general refused , asserting that he was under the command of the Diet of the German Confederation and not of the King of Prussia but of the regent of Germany . Wrangel proposed that , at the very least , any treaty concluded should be presented for ratification to the Frankfurt Parliament . The Danes rejected this proposal and negotiations were broken off . Prussia was now confronted on the one side by the German nation urging her clamorously to action , on the other side by the European powers threatening dire consequences should she persist . After painful hesitation , Frederick William chose what seemed the lesser of two evils , and , on 26 August , Prussia signed a convention at Malmö which yielded to practically all the Danish demands . The Holstein estates appealed to the German diet , which hotly took up their cause , but it was soon clear that the central government had no means of enforcing its views . In the end the convention was ratified at Frankfurt . The convention was essentially nothing more than a truce establishing a temporary " modus vivendi " . The main issues , left unsettled , continued to be hotly debated . In October , at a conference in London , Denmark suggested an arrangement on the basis of a separation of Schleswig from Holstein , which was about to become a member of a new German empire , with Schleswig having a separate constitution under the Danish crown . 1850 . In April 1850 , Prussia , which by now was war - weary , proposed a definitive peace on the basis of the " status quo ante bellum " and postponement of all questions as to mutual rights . To Palmerston the basis seemed meaningless and the proposed settlement would settle nothing . Nicholas I , openly disgusted with Frederick William ' s submission to the Frankfurt Parliament , again intervened . To him Duke Christian of Augustenborg was a rebel . Russia had guaranteed Schleswig to the Danish crown by the treaties of 1767 and 1773 . As for Holstein , if the King of Denmark could not deal with the rebels there , he himself would intervene as he had done in Hungary . The threat was reinforced by the menace of the European situation . Austria and Prussia were on the verge of war , and the sole hope of preventing Russia from entering such a war on the side of Austria lay in settling the Schleswig - Holstein question in a manner desirable to her . The only alternative , an alliance with the hated Napoleon Bonaparte ' s nephew , Louis Napoleon , who was already dreaming of acquiring the Rhine frontier for France in return for his aid in establishing German sea - power by the ceding of the duchies , was abhorrent to Frederick William . 1852 . The Protocol affirmed the integrity of the Danish federation as a " European necessity and standing principle " . Accordingly , the duchies of Schleswig ( a Danish fief ) and Holstein , and Lauenburg ( sovereign states within the German Confederation ) were joined by personal union with the King of Denmark . For this purpose , the line of succession to the duchies was modified , because Frederick VII of Denmark remained childless and hence a change in dynasty was in order . ( The originally conflicting protocols of succession between the duchies and Denmark would have stipulated that , contrary to the treaty , the duchies of Holstein and Lauenburg would have had heads of state other than the King of Denmark . ) Further , it was affirmed that the duchies were to remain as independent entities , and that Schleswig would have no greater constitutional affinity to Denmark than Holstein . This settlement did not resolve the issue , and only fifteen years passed before the Second Schleswig War in 1864 resulted in the incorporation of both duchies into the German Confederation , and later , in 1871 , into the German Empire .