In March and April 2021, before the 2022 Russian invasion of Ukraine, the Russian Armed Forces began massing thousands of personnel and military equipment near Russia's border with Ukraine and in Crimea, representing the largest mobilisation since the illegal annexation of Crimea in 2014. This precipitated an international crisis due to concerns over a potential invasion. Satellite imagery showed movements of armour, missiles, and heavy weaponry towards the border. The troops were partially withdrawn by June 2021, though the infrastructure was left in place. A second build-up began in October 2021, this time with more soldiers and with deployments on new fronts; by December over 100,000 Russian troops were massed around Ukraine on three sides, including Belarus from the north and Crimea from the south. Despite the Russian military build-ups, Russian officials from November 2021 to 20 February 2022 repeatedly denied that Russia had plans to invade Ukraine.

The crisis was related to the war in Donbas, itself part of the Russo-Ukrainian war, ongoing since February 2014. Intercepted phone conversations of Sergey Glazyev, a top advisor to Russian president Vladimir Putin, disclosed the specifics of the project Novorossiya to take over not just Crimea, but also the Donbas, Kharkiv, Odesa, and Zaporizhzhia regions of Ukraine, which Russia apparently aimed to annex following Crimea. The plan involved fomenting widespread unrest using pro-Russian agents on the ground, and then orchestrating uprisings that would announce rigged referendums about joining Russia, similar to the one that took place in Crimea on 16 March 2014. In December 2021, Russia advanced two draft treaties that contained requests for what it referred to as "security guarantees", including a legally binding promise that Ukraine would not join the North Atlantic Treaty Organization (NATO) and a reduction in NATO troops and materiel stationed in Eastern Europe, threatening unspecified military response if those demands were not met in full. NATO rejected these requests, and the United States warned Russia of "swift and severe" economic sanctions should it further invade Ukraine. The crisis was described by many commentators as one of the most intense in Europe since the Cold War.

On 21 February 2022, Russia officially recognised the two breakaway regions in eastern Ukraine, the Donetsk People's Republic and the Luhansk People's Republic, as independent states, and deployed troops to Donbas, in a move interpreted as Russia's effective withdrawal from the Minsk Protocol. The breakaway republics were recognised in the boundaries of their respective Ukrainian oblasts, although much of this territory was still held by Ukrainian government forces. On 22 February, Putin declared the Minsk agreements as invalid and the Federation Council unanimously authorised him to use military force in the territories. On the morning of 24 February, Putin announced that Russia was initiating a "special military operation" in the Donbas, and launched a full-scale invasion into Ukraine.

Background

Following the dissolution of the Soviet Union in 1991, Ukraine and Russia continued to retain close ties. Despite being a founding state of the CIS, Ukraine never signed the CIS Charter, therefore never becoming a formal member of the CIS.

In 1994, Ukraine, Russia, the UK and the US signed the Budapest Memorandum on Security Assurances, agreeing to abandon its nuclear arsenal in exchange for assurances from Russia, the United Kingdom, and the United States against threats or the use of force towards the territorial integrity or political independence of Ukraine. Five years later, Russia became a signatory of the Charter for European Security, where it "reaffirmed the inherent right of each and every participating State to be free to choose or change its security arrangements, including treaties of alliance, as they evolve".

Despite being recognised as an independent country since 1991, Ukraine continued to be perceived by Russian leadership as part of its sphere of influence due to its status as a former USSR constituent republic. In 2008, Russian president Vladimir Putin spoke out against Ukraine's membership in NATO. In 2009, Romanian analyst Iulian Chifu and his co-authors opined that in regard to Ukraine, Russia has pursued an updated version of the Brezhnev Doctrine, which dictates that the sovereignty of Ukraine cannot be larger than that of the Warsaw Pact's member states before the collapse of the Soviet sphere of influence during the late 1980s and early 1990s. In Putin's view, Russia's actions to placate the West in the early 1990s should have been met with reciprocity from the West, thus without NATO expansion along Russia's border.Remarks concerning Ukraine by Vladimir Putin. . RT. 21 February 2022. Minutes 41:17–42:58. "Because of the openness and goodwill, our goodwill, relations between Russia and the West were at a high level. Russia fulfilled all its obligations, we withdrew troops from Germany, from the central and European countries, and it made a huge contribution in overcoming the legacy of the Cold War. We consistently suggested all kinds of cooperation, including in the form of the NATO council and OECD. ... [When I asked], how would America see Russia joining NATO? ... How did Americans really look at this possibility? You can see it in their practical steps, in regard for a country: Open support of the terrorists in North Caucasus; Ignoring our demands and our concerns in the security area; Withdrawing from the arms treaties, and so on and so forth. It still begs the question, why? Why did they do that? What for? Okay, you don't want to see a friend in us, an ally in us. But why do you want to make an enemy out of us?"

Following months of Euromaidan protests, on 21 February 2014, pro-Russian Ukrainian president Viktor Yanukovych and parliamentary opposition leaders signed an agreement calling for an early election. The following day, Yanukovych fled Kyiv ahead of an impeachment vote that stripped him of his presidential authority. Leaders of the Russian-speaking eastern regions of Ukraine declared continued loyalty to Yanukovych, causing the 2014 pro-Russian unrest in the country. This unrest was fomented by Russia as part of a coordinated political and military campaign against Ukraine. This was followed by Russia's invasion and subsequent annexation of Crimea in March 2014 and the beginning of the Donbas war in April, with the creation of the Russia-backed quasi-states of the Donetsk and Luhansk People's Republics. The Minsk agreements allowed the fighting to subside in Donbas, leaving separatists in control of about a third of the region. This stalemate led to the war being labelled a "frozen conflict".

Beginning in 2019, Russia issued over 650,000 internal Russian passports to Donbas residents, which the Ukrainian government viewed as a step towards the annexation of the region. On 14 September 2020, Ukrainian president Volodymyr Zelenskyy approved a new national security strategy for the country, signaling Ukraine's intent to foster a stronger relationship with NATO "with the aim of membership in [the group]." On 24 March 2021, Zelenskyy signed Decree No. 117/2021 approving the government's strategy for the "de-occupation and reintegration" of Crimea, including Sevastapol. The decree complemented the activities of the already existing Crimean Platform while also mentioning other means for regaining control of the region, including through potential military force. The next day, Zelenskyy enacted the National Security and Defence Council's decision on Ukraine's military security strategy, protecting the country from external threats through deterrence, internal stability in times of crisis, and cooperation, particularly with the EU and NATO. The decree additionally described Russia as a "military adversary" which "carries out armed aggression against Ukraine... [and] uses military, political, economic, informational and psychological, space, cyber and other means that threaten [the] independence, state sovereignty and territorial integrity" of the country.

In Russia, Putin's close adviser Nikolai Patrushev was a leading figure in updating the country's national security strategy, published in May 2021. It states that Russia may use "forceful methods" to "thwart or avert unfriendly actions that threaten the sovereignty and territorial integrity of the Russian Federation." Russia has said that a possible Ukrainian accession to NATO and NATO enlargement in general threaten its national security. In turn, Ukraine and other European countries neighboring Russia have accused Putin of attempting to restore the Russian Empire/Soviet Union and of pursuing aggressive militaristic policies.

Shortly after the annexation of Crimea in 2014, Ukraine blocked the flow of the North Crimean Canal, which had supplied 85 percent of Crimea's water. Crimea's reservoirs were subsequently depleted and water shortages ensued; water was reportedly only being available for three to five hours a day in 2021. The New York Times cited senior American officials mentioning that securing Crimea's water supply could be an objective of a Russian invasion.

In July 2021, Putin published an essay titled On the Historical Unity of Russians and Ukrainians, in which he re-affirmed his view that Russians and Ukrainians were "one people". In response, American historian Timothy Snyder characterised Putin's ideas as imperialism while British journalist Edward Lucas described it as historical revisionism. Other observers have noted that the Russian leadership has a distorted view of modern Ukraine and its history. Some historians, including James Ellison and Michael Cox, contend that Putin became convinced by his government's active measures and ultimately believed Russian propaganda campaigns and false allegations of genocide in Donbas.

Initial tensions (March–April2021) First Russian military buildup On 21 February 2021, the Russian Defence Ministry announced the deployment of 3,000 paratroopers to the border for "large-scale exercises". The announcement was preceded by President Zelenskyy's decision on 2 February to implement recommendations from the country's National Security and Defence Council, which were intended to crackdown on Russian propaganda in Ukraine. Amongst the measures enacted by Zelenskyy were sanctions on Opposition Platform — For Life party People's Deputies Viktor Medvedchuk and Taras Kozak, and a national ban on multiple pro-Russian television channels, including 112 Ukraine, NewsOne, and ZIK. Medvedchuk, who also had alleged links to the banned media outlets, was a leading pro-Russian Ukrainian opposition politician and tycoon with close personal ties to Vladimir Putin. An analysis by Time published in February 2022 cited the event as the start of the Russian military buildup near Ukraine. According to an April 2023 investigative report by the Russian website Vertska, the banning of Medvedchuk's channels was the final catalyst for Putin deciding to take military action against Ukraine. The report further claimed that he made the decision in near-total secrecy between February and March 2021, with Russian businessman and close friend Yury Kovalchuk one of the very few people aware of Putin's plans at the time; the anti-Western Kovalchuk supposedly convinced Putin that he should act whilst Europe remained distracted by internal political divisions.