Montenegro and Serbia maintain diplomatic relations established in 1879. Both were part of the Kingdom of Yugoslavia (1918–1941) and later the Socialist Federal Republic of Yugoslavia (1945–1992). After Yugoslavia’s dissolution in 1991-1992, they formed the Federal Republic of Yugoslavia which was in 2003 reconstituted into the State Union of Serbia and Montenegro. The Montenegrin independence referendum in 2006, resulting in restoration of both countries' independence, marked a political divergence, but cultural ties between Serbia and Montenegro remain exceptionally strong.

The two countries have a deep historical and cultural connection, rooted in the centuries of intertwined history. The peoples of Serbia and Montenegro share common cultural traditions, including religion (majority in both countries adhere to the Serbian Orthodox Church) and language (vast majority in Serbia and relative majority in Montenegro speak Serbian as a mother tongue). About two million people in Serbia have partial or full ancestry from present-day Montenegro, mostly tracing back to the 18th and 19th centuries, vast majority of whom identify ethnically as Serb, though many (particularly first- or second-generation descendants from Montenegro) also claim a fairly strong Montenegrin regional identity. This closeness between Serbia and Montenegro causes sporadic debates on the Montenegrin ethnic and linguistic identity centering on a core question whether Montenegrins are essentially the same people as Serbs or distinct ethnicity.

History Medieval period

Duklja was a medieval South Slavic polity (10th–12th centuries) centred in modern-day Montenegro, around Lake Skadar and Zeta. Byzantine sources, such as those by John Skylitzes, referred to Duklja’s rulers (e.g., Stefan Vojislav) as "leaders of Serbs", indicating a Serb ethnic character. Duklja is often framed as a distinct proto-Montenegrin state by modern Montenegrin national narratives. Serbian historiography generally considers Duklja a Serb state, part of the medieval Serb political and cultural sphere, with its rulers and population integrated into the later Nemanjić state. Most historians agree that Duklja was at least partially a Serb state, particularly under the Vojislavljević dynasty, but its precise identity remains contested due to the complex ethnic and political landscape of the medieval Balkans.

By the late 11th and early 12th centuries, Duklja weakened due to internal strife and Byzantine interventions. Raška emerged as the leading Serbian state. Duklja, increasingly referred to as Zeta, was gradually absorbed into Raška’s sphere and by the late 12th century, Zeta was integrated into the Serbian Kingdom (proclaimed in 1217) and later the Serbian Empire under Stefan Dušan. Zeta was governed by Serbian nobles or royal heirs, such as Dušan’s son, Stefan Uroš, who was titled "King of Zeta". After Stefan Dušan’s death in 1355, the Serbian Empire fragmented. Zeta regained semi-independence under local dynasties like the Balšić noble family, who ruled as Serb lords but pursued their own policies.

By the 15th century, both Zeta and Serbia faced Ottoman expansion. Zeta, under the Crnojević noble family, maintained some independence longer than Serbia, which fell to the Ottomans in 1459. Zeta’s rulers sought Venetian and Hungarian support, diverging from Serbia’s path. The Crnojevićs still identified as Serbs, and Zeta’s Orthodox heritage aligned with Serbian traditions.

Ottoman rule The relations between Serbia and Montenegro from 16th to 18th centuries were influenced by geographical separation and limited direct interaction due to Ottoman rule dominance. During this period, "Montenegro" refers primarily to the region of Old Montenegro (rugged highlands around Cetinje), governed by the Petrović-Njegoš dynasty, while "Serbia" refers to areas under Ottoman control, particularly the Pashalik of Belgrade. At this time there was very little distinction between Serbs and Montenegrins, if any. The Metropolitanate of Montenegro and the Littoral, eparchy under constant jurisdiction of the Serbian Patriarchate of Peć, directly influenced the establishment of the Prince-Bishopric of Montenegro in 1697.Victoria Clark, Why angels fall: a journey through Orthodox Europe from Byzantium to Kosovo, p. 93Robert Bideleux, Ian Jeffries, A history of eastern Europe: crisis and change, p. 86 While nominally under Ottoman suzerainty, Montenegro’s mountainous terrain and tribal organization allowed it to maintained de facto autonomy and to resist direct Ottoman control. Montenegro’s autonomy allowed it to act as a symbol of resistance against Ottoman rule, inspiring Serbs in Ottoman-controlled Serbia.

XIX century

Serbia, following the Serbian Revolution, gained partial autonomy under Miloš Obrenović, formalized by the Ottoman decrees of 1830 and 1833. Montenegro, under the rule of the Petrović-Njegoš dynasty, operated as a de facto independent theocratic state, though it lacked international recognition. While both shared anti-Ottoman goals, direct military or political coordination was minimal due to geographical separation and Montenegro’s isolation. However, Montenegro’s rulers expressed solidarity with uprisings during the Serbian Revolution, and Petar I Petrović-Njegoš corresponded with leader of the First Serbian Uprising, Karađorđe.

From 1830s through 1860s, Serbia pursued cautious modernization and diplomacy within the Ottoman framework, while Montenegro adopted a more militant, confrontational approach. This led to occasional tensions, as Serbia was wary of provoking the Ottomans, whereas Montenegro openly defied them. Montenegro occasionally sought Serbia’s support in its conflicts with the Ottomans, such as during clashes in the 1840s. However, Serbian cautious diplomacy under Ottoman oversight limited its ability to openly aid Montenegro.

Since the beginning of the 1860s dynamics changed as there were closer alignment between the two and against Ottomans. Prince Mihailo Obrenović of Serbia sought to form a Balkan alliance against the Ottomans, including Montenegro. In 1866–1868, Serbia and Montenegro signed secret agreements to coordinate anti-Ottoman activities, alongside other Balkan states like Greece and Romania. These plans aimed at a joint uprising but were disrupted by Mihailo Obrenović’s assassination in 1868. Montenegro played a leading role in supporting the Herzegovina Uprising, a revolt of Serbs against Ottoman rule in Bosnia and Herzegovina. Serbia joined the uprising in 1876, declaring war on the Ottomans alongside Montenegro. This marked the closest military cooperation between the two states in the 19th century. Serbia and Montenegro fought together against the Ottomans in Serbian–Ottoman Wars from 1876 to 1878, with Montenegro achieving significant victories (such as the Battle of Vučji Do). Their joint efforts strengthened their diplomatic position as the Great Powers intervened to resolve the conflict culminating at the Congress of Berlin in 1878, when both Serbia and Montenegro were granted formal recognition of independence.

However, Austria-Hungary’s growing influence over Serbia during the 1880s and 1890s created a divergence in foreign policy, with Montenegro aligning more closely with Russia.

Balkan Wars The 1903 coup in Serbia, which overthrow house of Obrenović and brought King Peter I of house of Karađorđević to power, gave new dynamics to mutual relations. The new ruler adopted a more assertive nationalist policy, aligning Serbia closer to Russia and Montenegro.Christopher Clark, The Sleepwalkers, pp. 3–12 This shift strengthened Serbia-Montenegro relations, as both states shared anti-Austrian and anti-Ottoman goals. The Bosnian Crisis, when Austria-Hungary annexed Bosnia and Herzegovina, galvanized both Serbia and Montenegro. Montenegro supported Serbia’s protests against the annexation.

Both states harbored ambitions to lead South Slavic liberation movements. Serbia, with its larger population and resources, began to see itself as the nucleus of a future South Slavic state, which sometimes caused friction with Montenegro’s independent-minded ruler, King Nicholas I, who harbored ambitions to lead such a state. In March 1912, Serbia and Montenegro signed a formal alliance treaty, committing to mutual military support against the Ottoman Empire. The treaty outlined plans for coordinated action to liberate South Slavic and other Balkan territories, with provisions for dividing conquered territories.

Montenegro initiated the First Balkan War on October 8, 1912, by declaring war on the Ottoman Empire. In a prelude to the war, Serbia, Montenegro, Bulgaria, and Greece formed the Balkan League, a military alliance aimed at expelling the Ottoman Empire from the Balkans.The Ear Correspondence of Leon Trotsky: Hall, The Balkan Wars, 1912–13, 1980, p. 221 Montenegro’s early offensive, particularly in the Sanjak of Novi Pazar and Kosovo, set the stage for broader Balkan League operations. Serbia joined the war shortly after, launching successful campaigns in Kosovo, Macedonia, and northern Albania. Serbian and Montenegrin forces coordinated closely, particularly in the capture of key Ottoman strongholds like Kumanovo and Bitola. The Treaty of London ended the war, with Serbia and Montenegro securing substantial territorial gains from the Ottoman Empire.

The Second Balkan War erupted when Bulgaria, dissatisfied with the territorial division, attacked Serbia and Greece. Montenegro sided with Serbia and fought together against, with Serbia leading major offensives. Montenegro’s contribution was smaller due to its limited resources, but it provided crucial support in maintaining pressure on Bulgarian forces in Battle of Bregalnica. The war ended with a victory for Serbia, Montenegro, Greece, and Romania against Bulgaria. Serbia gained additional territories in Macedonia, while Montenegro secured parts of the Sanjak and northern Kosovo. The creation of an independent Albania, enforced by the Great Powers (particularly Austria-Hungary), frustrated both Serbia and Montenegro, as it limited or, in case of Serbia, entirely blocked, their access to the Adriatic Sea. Montenegro was forced to withdraw from Shkodër, and Serbia lost northern Albanian territories it had occupied. This shared grievance strengthened their anti-Austrian stance.

World War I

Serbia faced direct aggression from Austria-Hungary following the assassination of Archduke Franz Ferdinand in 1914 in Sarajevo, which triggered the World War I. Montenegro immediately declared solidarity with Serbia, viewing the conflict as an extension of their shared struggle for South Slavic liberation. The Austrians then continued pushing their offensive south.Mitrović, Andrej (2007) Serbia's great war, 1914-1918 Purdue University Press, West Lafayette, Indiana, page 155,

Serbia and Montenegro coordinated their military strategies early in the war. Serbia focused on defending its territory against Austro-Hungarian invasions, while Montenegro engaged Austro-Hungarian forces along its borders, particularly in the Sanjak and Bosnia-Herzegovina. Montenegro’s army, though small, played a certain role in tying down Austro-Hungarian troops. In 1914, Serbia achieved significant victories, such as the Battle of Cer and the Battle of Kolubara, repelling Austro-Hungarian offensives.