In March and April 2021, the Russian Armed Forces began massing thousands of personnel and military equipment near Russia's border with Ukraine and in Crimea, representing the largest mobilization 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. The troops were partially withdrawn by June 2021, though the infrastructure was left in place.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. The 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 March 16 that year. 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, and threatened 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 recognized 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 recognized 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. On the same day, the Federation Council unanimously authorised the use of 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. In 1994, Ukraine agreed to abandon its nuclear arsenal and signed the Budapest Memorandum on Security Assurances on the condition that Russia, the United Kingdom, and the United States issue an assurance against threats or use of force to the territorial integrity or political independence of Ukraine. Five years later, Russia was one of the signatories 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 recognized as an independent country since 1991, as a former USSR constituent republic, Ukraine was perceived by the leadership of Russia as part of its sphere of influence. 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 prior to the collapse of the Soviet sphere of influence during the late 1980s and early 1990s. This view is built upon the premise that Russia's actions to placate the West in the early 1990s should have been met with reciprocity from the West, without NATO expansion along Russia's border.