The defense of Van (Armenian: Վանի հերոսամարտ, romanized: Vani herosamart) and in Russian Van operation (Russian: Ванская операция, romanized: Vanskaya operatsia) was the armed resistance of the Armenian population of Van and Russian army against the Ottoman Empire's attempts to massacre the Ottoman Armenian population of the Van Vilayet in the 1915 Armenian genocide. Several contemporaneous observers and later historians have concluded that the Ottoman government deliberately instigated an armed Armenian resistance in the city and then used this insurgency as the main pretext to justify beginning the deportation and slaughter of Armenians throughout the empire. Witness reports agree that the Armenian posture at Van was defensive and an act of resistance to massacre. The self-defense action is frequently cited in Armenian genocide denial literature; it has become "the alpha and omega of the plea of 'military necessity'" to excuse the genocide and portray the persecution of Armenians as justified.

During the late Ottoman period, Van was an important center of Armenian cultural, social, and economic life. Khrimian Hayrik established a printing press in Van, and thereafter launched Vaspurakan Ardzvi (Eagle of Vaspourakan), which was the first periodical publication in Armenia. In 1885, the Armenakan party was established in the city of Van. Soon after, the Hnchak and Dashnak parties, whose missions were the overthrow of the Ottoman rule in Eastern Anatolia (Six Vilayets), established branches in the city.

Throughout 1895–96 Armenians in the Ottoman Empire suffered a wave of violence commonly known as the Hamidian massacres. While Van largely avoided the massacres in 1895, the Ottomans sent a military expedition in June 1896. The Armenians of Van were initially able to defend themselves, but upon agreeing to disarm in exchange for safety, massacres continued, ending in the deaths of over 20,000 Armenians.

The pre-war (World War I) demographic statistics of the Van Province, Ottoman Empire had different values based on different sources. In 1914, Armenians lived on the shores of Lake Van. The major Armenian inhabited localities were the city of Van (consisting of three sub-sections which were Havasor (Gürpınar), Timar (Gedikbulak) and Archak (Erçek)). Armenians also lived in the district of Erciş (Artchesh, Akants) which was in the northern part of the province, as well as the districts of Çatak (Shatakh), Başkale (Bashkaleh) and Bahçesaray (Moks) in the southern part of the province. In an 1890 census, there were 79,998 Armenians living in the province. That same census showed the percentage Armenian population located in the city of Van to be 35%, and the district Armenian populations to be 64% in Ercis, 37% in Catak, 18% in Bashkale, and 48% in Bahcesaray. The 1912 local Patriarch statistic stated that the Armenian population was 110,000. The original 1914 Ottoman census stated that Armenian population was 67,797 and Muslim population was 179,422. However, the 1914 official census was challenged both on Armenian and Muslim population sizes. It was found that the original 1914 Ottoman statistics claimed to be under-representative for the children, and the corrected values for the Van province were stated as 313,000 Muslim, 130,000 Armenian (25%), and approximately 65,000 Syrian, Chaldean, Nestorian and others. Population estimates for the city of Van proved more difficult to analyze. Extensive population movements in and around the city happened due to the deterioration of the economic and political situation before World War I. Ottoman population count at the time recorded 79,000 Muslims and 34,000 Armenians in the city of Van, including the immediate surrounding areas. The city of Van's Armenian population was estimated to be about 30,000 people in the fall of 1914.

On 30 October 1914, after an exchange of fire during the pursuit of Goeben and Breslau, the Ottoman Empire entered World War I. The province of Van was positioned between Persia and the Caucasus, and the most accessible routes that linked Persia, Russia, Mesopotamia and Anatolia lay through this province. giving Van high strategic value as a consequence of its location.

The first engagement of the Caucasus Campaign took place on 2 November 1914 with the Bergmann Offensive. The Russians collected victories along the Kara Kilise (renamed Karaköse in November 1919, present day Ağrı) – Beyazit (Doğubayazıt) line. Beyazit was located in the north of the Province of Van, and Armenian volunteers provided aid as auxiliary forces to Russian regiments while capturing these regions.

During December 1914, Nicholas II of Russia visited the Caucasian front. In the presence of the head of the Armenian Church and alongside Alexander Khatisyan president of the Armenian National Bureau in Tiflis, Nicholas II stated: "Armenians from all countries are hurrying to enter the ranks of the glorious Russian Army, and with their blood, to serve the victory of the Russian Army... Let the Russian flag wave freely over the Dardanelles and the Bosporus, let all the peoples (Christians) remaining under the Turkish yoke receive freedom through your will. Let the Armenian people of Turkey, who have suffered for the faith of Christ, receive resurrection for a new and free life...". Ottoman War Minister Enver Pasha wanted to encircle the Russian forces between Sarikamish and Ardahan. The Battle of Sarikamish (29 December 1914 – 4 January 1915) was a disastrous defeat for Enver Pasha. For their role in this success, the Armenian volunteers received credit as natives of the region, well adjusted to the climate, familiar with every road and mountain path, and motivated to wage fierce and resolute combat. The Armenian volunteers were small, mobile units, well adapted to semi-guerrilla warfare. They worked efficiently as scouts, though they also took active roles in many conventional engagements. Armenian volunteers challenged the Ottoman operations during critical times: "the delay they caused, enabled the Russian Caucasus Army to concentrate sufficient force around Sarikamish". After he returned to Constantinople, Enver blamed his defeat on Armenians living in the region, for actively siding with Russia.

On 11 December 1914 Ottoman preparations for their Persian Campaign began with Enver Pasha's order to form a provisional force that would be channeled into the planned theatre using the roads in Van Province. Whilst Turkish preparations were underway, the Russians transferred Armenian General Tovmas Nazarbekian to the Russian-occupied Persian Azerbaijan. Theodore G. Chernozubov's Persian Cossack Brigade had been operationally deployed in Persia since 1906. During this period, the Ottoman authorities distributed 24,000 rifles to Kurds in Persia and the Van province.