The First Republic of Armenia, officially known at the time of its existence as the Republic of Armenia, was the first modern Armenian state since the loss of Armenian statehood in the Middle Ages.

The republic was established in the Armenian-populated territories of the disintegrated Russian Empire, known as Eastern Armenia or Russian Armenia. The leaders of the government came mostly from the Armenian Revolutionary Federation (ARF or Dashnaktsutyun). The First Republic of Armenia bordered the Democratic Republic of Georgia to the north, the Ottoman Empire to the west, Persia to the south, and the Azerbaijan Democratic Republic to the east. It had a total land area of roughly 70,000 km², and a population of 1.3 million.

The Armenian National Council declared the independence of Armenia on 28 May 1918. From its very onset, Armenia was plagued with a variety of domestic and foreign issues. A humanitarian crisis emerged from the aftermath of the Armenian genocide as hundreds of thousands of Armenian refugees from the Ottoman Empire were forced to settle in the fledgling republic. Lasting two and a half years in existence, the Republic of Armenia became involved in several armed conflicts with its neighbors, caused by overlapping territorial claims. By late 1920, the nation was partitioned between the Turkish Nationalist forces and Russian Red Army. The First Republic, along with the Republic of Mountainous Armenia which repelled the Soviet invasion until July 1921, ceased to exist as an independent state, superseded by the Armenian Soviet Socialist Republic that became part of the Soviet Union in 1922. After the fall of the Soviet Union, the republic regained its independence as the current Republic of Armenia in 1991.

The Russian offensive during the Caucasus Campaign of World War I, the subsequent occupation, and the creation of a provisional administrative government gave hope for ending Ottoman Turkish rule in Western Armenia. With the help of several battalions of Armenians recruited from the Russian Empire, the Russian army had made progress on the Caucasus Front, advancing as far as the city of Erzurum in 1916. The Russians continued to make considerable advances even after the toppling of Tsar Nicholas II in February 1917.

In March 1917, the spontaneous revolution that toppled Tsar Nicholas and the Romanov dynasty established a caretaker administration, known as the Provisional Government. Shortly after, the Provisional Government replaced Grand Duke Nicholas' administration in the Caucasus with the five-member Special Transcaucasian Committee, known by the acronym Ozakom. The Ozakom included Armenian Democrat Mikayel Papadjanian, and was set to heal wounds inflicted by the old regime. In doing so, Western Armenia was to have a general commissar and was to be subdivided into the districts of Trebizond, Erzerum, Bitlis, and Van. The decree was a major concession to the Armenians: Western Armenia was placed under the central government and through it under immediate Armenian jurisdiction. Dr. Hakob Zavriev would serve as the assistant for civil affairs and he in turn would see to it that most civil officials were Armenian.

In October 1917, the Bolsheviks seized power from the Provisional Government and announced that they would be withdrawing troops from both the Western and Caucasus fronts. The Armenians, Georgians, and Muslims of the Caucasus all rejected the Bolsheviks' legitimacy.

On 5 December 1917, the Ottoman Empire and the Transcaucasian Commissariat signed the armistice of Erzincan, ending armed conflict. After the Bolshevik seizure of power, a multinational congress of Transcaucasian representatives met to create a provisional regional executive body known as the Transcaucasian Seim. The Commissariat and the Seim were heavily encumbered by the pretense that the South Caucasus formed an integral unit of a non-existent Russian democracy. The Armenian deputies in the Seim were hopeful that the anti-Bolshevik forces in Russia would prevail in the Russian Civil War and rejected any idea of separating from Russia. In February 1918, the Armenians, Georgians and Muslims had reluctantly joined to form the Transcaucasian Federation, but disputes among the three groups continued as unity began to falter.

On 3 March 1918, Russia followed the armistice of Erzincan with the Treaty of Brest-Litovsk and left the war. It ceded territory from 14 March to April 1918, when a conference was held between the Ottoman Empire and the delegation of the Seim. Under the Treaty of Brest-Litovsk, the Russians allowed the Turks to retake the Western Armenian provinces, as well as to take over the provinces of Kars, Batum, and Ardahan.

In addition to these provisions, a secret clause obligated the Armenians and Russians to demobilize their forces in both western and eastern Armenia.Having killed and deported most of Armenians of Western Armenia during the Armenian genocide, the Ottoman Empire intended to eliminate the Armenian population of Eastern Armenia.Shortly after the signing of Brest-Litovsk the Turkish army began its advance, taking Erzurum in March and Kars in April, which the Transcaucasian government of Nikolay Chkheidze had ordered soldiers to abandon. Beginning on 21 May, the Ottoman army moved ahead again.

On 11 May 1918, a new peace conference opened at Batum. At this conference, the Ottomans extended their demands to include Tiflis, as well as Alexandropol and Echmiadzin, which they wanted for a railroad to be built to connect Kars and Julfa with Baku. The Armenian and Georgian members of the Republic's delegation began to stall.

On 26 May 1918, Georgia declared independence; on 28 May, it signed the Treaty of Poti, and received protection from Germany. The following day, the Muslim National Council in Tiflis announced the establishment of the Democratic Republic of Azerbaijan.

Having been abandoned by its regional allies, the Armenian National Council, based in Tiflis and led by Russian Armenian intellectuals who represented Armenian interests in the Caucasus, declared its independence on May 28. It dispatched Hovhannes Kajaznuni and Alexander Khatisyan, both members of the ARF, to Yerevan to take over power and issued the following statement on 30 May (retroactive to 28 May):

In view of the dissolution of the political unity of Transcaucasia and the new situation created by the proclamation of the independence of Georgia and Azerbaijan, the Armenian National Council declares itself to be the supreme and only administration for the Armenian provinces. Because of the certain grave circumstances, the national council, deferring until the near future the formation of an Armenian National government, temporarily assumes all governmental functions, in order to take hold the political and administrative helm of the Armenian provinces.

Meanwhile, the Turks had taken Alexandropol and were intent on eliminating the center of Armenian resistance based in Yerevan. The Armenians were able to stave off total defeat and delivered crushing blows to the Turkish army in the battles of Sardarapat, Karakilisa and Abaran.

The Republic of Armenia had to sue for negotiations at the Treaty of Batum, which was signed in Batum on 4 June 1918. It was Armenia's first treaty. After the Ottoman Empire took vast swathes of territory and imposed harsh conditions, the new republic was left with 10,000 square kilometers.