In February and March 2014, Russia invaded and subsequently annexed the Crimean Peninsula from Ukraine. This event took place in the aftermath of the Revolution of Dignity and is part of the wider Russo-Ukrainian War.

On 2223 February 2014, Russian president Vladimir Putin convened an all-night meeting with security service chiefs to discuss assisting the deposed Ukrainian president Viktor Yanukovych with leaving the country. At the end of the meeting, Putin 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 (parliament) of Crimea and captured strategic sites across Crimea. This led to the installation of the pro-Russian Sergey Aksyonov government in Crimea, the Crimean status referendum and the declaration of Crimea's independence on 16 March 2014. Russia formally incorporated Crimea as two Russian federal subjectsthe Republic of Crimea and the federal city of Sevastopol on 18 March 2014. Following the annexation, Russia escalated its military presence on the peninsula and made nuclear threats to solidify the new status quo on the ground.Ukraine and many other countries condemned the annexation and consider it to be a violation of international law and Russian-signed agreements safeguarding the territorial integrity of Ukraine, including the 1991 Belavezha Accords that established the Commonwealth of Independent States, the 1975 Helsinki Accords, the 1994 Budapest Memorandum on Security Assurances and the 1997 Treaty on friendship, cooperation and partnership between the Russian Federation and Ukraine. The annexation led to the other members of the then-G8 suspending Russia from the group and then introducing a first round of sanctions against the country. The United Nations General Assembly also rejected the referendum and annexation, adopting a resolution affirming the "territorial integrity of Ukraine within its internationally recognised borders". The UN resolution also "underscores that the referendum having no validity, cannot form the basis for any alteration of the status of [Crimea]" and called upon all states and international organizations not to recognize or to imply the recognition of Russia's annexation. In 2016, the UN General Assembly reaffirmed non-recognition of the annexation and condemned "the temporary occupation of part of the territory of Ukrainethe Autonomous Republic of Crimea and the city of Sevastopol".The Russian government opposes the "annexation" label, with Putin defending the referendum as complying with the principle of the self-determination of peoples.

Crimea ( (listen) kry-MEE-ə) is a peninsula in Eastern Europe. It is situated along the northern coast of the Black Sea, and has a population of 2.4 million, made up mostly of ethnic Russians with significant Ukrainian and Crimean Tatar minorities. The peninsula is almost entirely surrounded by both the Black Sea and the smaller Sea of Azov; it is located south of Kherson Oblast in Ukraine, to which it is connected by the Isthmus of Perekop, and west of Krasnodar Krai in Russia, from which it is separated by the Strait of Kerch though linked by the Crimean Bridge since 2018. The Arabat Spit is located to its northeast, a narrow strip of land that separates a system of lagoons named Sivash from the Sea of Azov. Across the Black Sea to the west lies Romania and to the south is Turkey.

Crimea (or the Tauric Peninsula, as it was called from antiquity until the early modern period) has historically been at the boundary between the classical world and the Pontic–Caspian steppe. Its southern fringe was colonised by the Greeks and then ruled by the Persians followed by the Roman Empire, the Byzantine Empire, and finally successor states including the Empire of Trebizond and Principality of Theodoro. During the entirety of this period the urban areas were Greek-speaking and eventually eastern Christian (Eastern Orthodox). During the collapse of the Byzantine state some cities fell to its creditor, the Republic of Genoa, until eventually all were absorbed by the rapidly rising Ottoman Empire. Throughout this time the interior was occupied by a changing cast of invading steppe nomads and empires, such as the Cimmerians, Scythians, Sarmatians, Crimean Goths, Alans, Bulgars, Huns, Khazars, Kipchaks, Mongols, and the Golden Horde. Crimea and adjacent territories were united in the Crimean Khanate, a sometime dependency of the Ottomans, during the 15th to 18th century, and often raided south Russia for slaves.

In 1783, Crimea was annexed by the Russian Empire as the result of the Russo-Turkish War (1768–1774). Following the Russian Revolution of 1917, Crimea became an autonomous republic within the Russian SFSR in the Soviet Union. During World War II, Crimea was downgraded to the Crimean Oblast and the entirety of one of its indigenous populations, the Crimean Tatars, were deported to Central Asia, an act that was formally recognized as a genocide by Ukraine and three other countries between 2015 and 2019. In 1954, the Soviet Union transferred Crimea to the Ukrainian SSR from the Russian SFSR. The transfer to Ukraine was made by Soviet leader Nikita Khrushchev. The year 1954 happened to mark the 300th anniversary of the Treaty of Pereyaslav, which was signed in 1654 by representatives of the Ukrainian Cossack Hetmanate and Tsar Alexis of Russia. With the collapse of the Soviet Union, Ukraine was reestablished as an independent state in 1991, and most of the peninsula was reorganized as the Autonomous Republic of Crimea, and the city of Sevastopol retained its special status within Ukraine. The 1997 Partition Treaty on the Status and Conditions of the Black Sea Fleet partitioned the former Soviet Black Sea Fleet and allowed Russia to continue basing its fleet in Crimea: both the Ukrainian Naval Forces and Russia's Black Sea Fleet were to be headquartered in Sevastopol. Ukraine extended Russia's lease of the naval facilities under the 2010 Kharkiv Pact in exchange for further discounted natural gas.

The status of Crimea is disputed. In late February 2014, following the Revolution of Dignity that ousted the Ukrainian president, Viktor Yanukovych, Russian troops were deployed to Crimea, occupying government buildings. The Republic of Crimea declared its independence from Ukraine following a disputed referendum on 16 March, deemed illegal by Ukraine and most countries, which was held on the issue of reunification with Russia; its official results showed over 90% support for reunification, but the vote was boycotted by many loyal to Ukraine. Russia formally annexed Crimea on 18 March, incorporating the Republic of Crimea and the federal city of Sevastopol as the 84th and 85th federal subjects of Russia. Despite its annexation, Crimea was considered by most countries of the world in a UN resolution of March 2014 to remain part of Ukraine.