Ryssland placerar ut luftvärn på Krimhalvön

The S-400 Triumf (Russian: C-400 «Триумф», Triumph; NATO reporting name: SA-21 Growler), previously known as S-300PMU-3, is an anti-aircraft weapon system developed by Russia's Almaz Central Design Bureau in the 1990s as an upgrade of the S-300 family. It has been in service with the Russian Armed Forces since 2007. The S-400 uses three different missiles to cover its entire performance envelope. These are the extremely long range 40N6, long range 48N6 and medium range 9M96 missile.

The Ukrainian territory of Crimea was annexed by the Russian Federation on 18 March 2014. Since then, the peninsula has been administered as the de facto Crimean Federal District, constituting two Russian federal subjects—the Republic of Crimea and the city of Sevastopol. The annexation followed a military intervention by Russia in Crimea, which took place in the aftermath of the 2014 Ukrainian revolution and was part of wider unrest across southern and eastern Ukraine. On 22–23 February, Russian President Vladimir Putin convened an all-night meeting with security services chiefs to discuss extrication of the deposed Ukrainian president, Viktor Yanukovych, and at the end of that meeting Putin had remarked that "we must start working on returning Crimea to Russia." On 23 February pro-Russian demonstrations were held in the Crimean city of Sevastopol. On 27 February masked Russian troops without insignia took over the Supreme Council of Crimea, and captured strategic sites across Crimea, which led to the installation of the pro-Russian Aksyonov government in Crimea and the declaration of Crimea's independence. Ukraine considers the annexation to be a violation of international law and agreements by Russia, including Agreement on Establishing the Commonwealth of Independent States in 1991, Helsinki Accords, Treaty on the Non-Proliferation of Nuclear Weapons of 1994 and Treaty on friendship, cooperation and partnership between the Russian Federation and Ukraine. The event was condemned by many world leaders as an illegal annexation of Ukrainian territory, in violation of the 1994 Budapest Memorandum on sovereignty and territorial integrity of Ukraine, signed by Russia. It led to the other members of the then G8 suspending Russia from the group, then introducing the first round of sanctions against the country. The United Nations General Assembly also rejected the vote and annexation, adopting a non-binding resolution affirming the "territorial integrity of Ukraine within its internationally recognised borders". Following a Crimean referendum, held on 16 March 2014, in which separation was favored by a large majority of voters, the UN resolution also "underscores that the referendum having no validity, cannot form the basis for any alteration of the status of [Crimea]." The resolution calls upon all States and international organizations not to recognize or to imply the recognition of Russia's annexation. The Russian Federation opposes the "annexation" label, with Putin defending the referendum as complying with the principle of self-determination of peoples. In July 2015, Russian prime minister Dmitry Medvedev said that Crimea had been fully integrated into Russia.