Montenegro suffered severely in World War I. Shortly after Austria-Hungary declared war on Serbia (28 July 1914), Montenegro lost little time in declaring war on the Central Powers – on Austria-Hungary in the first instance – on 6 August 1914, despite Austrian diplomacy promising to cede Shkoder to Montenegro if it remained neutral. For purposes of coordination in the fight against the enemy army, Serbian General Bozidar Jankovic was named head of High Command of both Serbian and Montenegrin armies. Montenegro received 30 artillery pieces and financial help of 17 million dinars from Serbia. France contributed a colonial detachment of 200 men located in Cetinje at the beginning of war, as well as two radio-stations – located on top of Mount Lovćen and in Podgorica. Until 1915 France supplied Montenegro with necessary war material and food through the port of Bar, which was blockaded by Austrian battleships and submarines. In 1915 Italy took over this role, running supplies unsuccessfully and irregularly across the line Shengjin-Bojana-Lake Skadar, an unsecured route because of constant attacks by Albanian irregulars organised by Austrian agents. Lack of vital materials eventually led Montenegro to surrender.

Austria-Hungary dispatched a separate army to invade Montenegro and to prevent a junction of the Serbian and Montenegrin armies. This force, however, was repulsed, and from the top of the strongly fortified Lovćen, the Montenegrins carried on the bombardment of Kotor held by the enemy. The Austro-Hungarian army managed to capture the town of Pljevlja while on the other hand the Montenegrins took Budva, then under Austrian control. The Serbian victory at the Battle of Cer (15–24 August 1914) diverted enemy forces from Sandjak, and Pljevlja came into Montenegrin hands again. On August 10, 1914, the Montenegrin infantry delivered a strong attack against the Austrian garrisons, but they did not succeed in making good the advantage they first gained. They successfully resisted the Austrians in the second invasion of Serbia (September 1914) and almost succeeded in seizing Sarajevo. With the beginning of the third Austro-Hungarian invasion, however, the Montenegrin army had to retire before greatly superior numbers, and Austro-Hungarian, Bulgarian and German armies finally overran Serbia (December 1915). However, the Serbian army survived, and led by King Peter I of Serbia, started retreating across Albania. In order to support the Serbian retreat, the Montenegrin army, led by Janko Vukotic, engaged in the Battle of Mojkovac (6–7 January 1916). Montenegro also suffered a large scale invasion (January 1916) and for the remainder of the war remained in the possession of the Central Powers. See Serbian Campaign (World War I) for details. The Austrian officer Viktor Weber Edler von Webenau served as the military governor of Montenegro between 1916 and 1917. Afterwards Heinrich Clam-Martinic filled this position.

King Nicholas fled to Italy (January 1916) and then to France; the government transferred its operations to Bordeaux. Eventually the Allies liberated Montenegro from the Austrians. A newly convened National Assembly of Podgorica, accused the King of seeking a separate peace with the enemy and consequently deposed him, banned his return and decided that Montenegro should join the Kingdom of Serbia on December 1, 1918. A part of the former Montenegrin military forces still loyal to the King started a rebellion against the amalgamation, the Christmas Uprising (7 January 1919).