He gave Montenegro its first constitution in 1905 following pressure from a population eager for more freedom. He also introduced west-European style press freedom and criminal law codes. In 1906, he introduced Montenegrin currency, the perper. On 28 August 1910, during the celebration of his jubilee, he assumed the title of king, in accordance with a petition from the Skupština. He was at the same time gazetted field-marshal in the Russian army, an honor never previously conferred on any foreigner except the Duke of Wellington. When the Balkan Wars broke out in 1912 King Nikola was one of the most enthusiastic of the allies. He wanted to drive the Ottomans completely out of Europe. He defied the Concert of Europe and captured Scutari after a siege, despite the fact that they blockaded the whole coast of Montenegro. Again in the Great War which began in 1914 he was the first to go to Serbia's aid to repel the Austro-Hungarian forces from the Balkan Peninsula.
In January 1916, after the defeat of Serbia, Montenegro was also conquered by Austria-Hungary, and the King fled to Italy and then to France. The government transferred its operations to Bordeaux. After the end of the First World War, a meeting in Podgorica voted to depose Nikola and annex Montenegro to Serbia. A few days later, Serbia (including Montenegro) merged with the former South Slav territories of Austria-Hungary to form the Kingdom of Serbs, Croats and Slovenes, which was renamed Yugoslavia in 1929. Nikola, who was in exile in France, continued to claim the throne until his death in Antibes in 1921. He was buried in Italy. In 1989, the remains of Nikola, his queen Milena, and two of their twelve children were re-buried in Montenegro.