Petar Petrović Njegoš, an influential vladika, reigned in the first half of the 19th century. In 1851 Danilo Petrović Njegoš became vladika, but in 1852 he married and renounced his ecclesiastical character, assuming the title of knjaz (Prince) Danilo I, and transformed his land into a secular principality. Following the assassination of Danilo by Todor Kadić in Kotor, in 1860, the Montenegrins proclaimed Nicholas I as his successor on August 14 of that year. In 1861–1862, Nicholas engaged in an unsuccessful war against the Ottoman Empire. Under Nicholas I the country was also granted its first constitution (1905) and was elevated to the rank of kingdom in 1910.

Following the Herzegovinian Uprising, partly initiated by his clandestine activities, he yet again declared war on Turkey. The Serbia joined Montenegro, but it was defeated by Turkish forces that same year. Russia now joined in and decisively routed the Turks in 1877–78. The Treaty of San Stefano (March 1878) was highly advantageous to Montenegro, as well as Russia, Serbia, Romania and Bulgaria. However, the gains were somewhat trimmed by the Treaty of Berlin (1878). In the end Montenegro was internationally recognized as an independent state, its territory was effectively doubled by the addition of 4,900 square kilometres (1,900 sq mi), the port of Bar and all the waters of Montenegro were closed to warships of all nations; and the administration of the maritime and sanitary police on the coast was placed in the hands of Austria.