Nikola became sovereign prince of Montenegro on the assassination of his uncle Danilo II in 1860. The country was embroiled in a series of wars with the Ottoman Empire between 1862 and 1878. However, the independence of Montenegro was recognised by all other countries at the Congress of Berlin in 1878 and in the succeeding decades Montenegro enjoyed considerable prosperity and stability. Education, communications and the army expanded greatly (the latter with support from Imperial Russia). Nikola 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.
The Montenegrin parliament declared Nikola king in 1910. After the First World WarMontenegro united with the Kingdom of Serbia, as well as with other south Slav lands, to form the Kingdom of Serbs, Croats and Slovenes, which was renamed Yugoslavia in 1929.
Nikola went into exile in 1918, but continued to claim the throne until his death in France three years later. 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. One of his daughters, Anastasia, was married to the World War One general, Grand Duke Nicholas Nikolayevich of Russia. Another of Nikola and Milena’s children was Queen Elena of Italy, wife of King Victor Emmanuel III of Italy. Many of his daughters were married off to various princes and kings, giving Nikola the nickname "the father-in-law of Europe".
In the intervening years before his appointment as King, Peter had served with the French during the unsuccessful Franco-Prussian War of 1870-71 and, in 1875, joined the Bosnian insurrection against the Ottomans.
The first years of King Peter's reign saw reforms to the constitution, the army and the school system, as well as improvements to the system of agriculture.
With Serbia's epic defeat towards the close of 1915 at the hands of combined Austro-Hungarian, German and Bulgarian forces led by Mackensen, King Peter had to be carried (along with similarly ill Putnik) through the wintry mountains of Albania to Corfu.