On 18 March 2014 the Russian Federation formally annexed the Ukrainian territory of Crimea. Ukraine considers the annexation a violation of international law and the event was condemned by many world leaders as illegal. Subsequently, the other members of the then G8 suspended Russia from the group and introduced a first round of sanctions against Russia. The United Nations General Assembly also rejected the annexation, adopting a non-binding resolution affirming the “territorial integrity of Ukraine within its internationally recognised borders”. The resolution also “underscores that the referendum [in Crimea], having no validity, cannot form the basis for any alteration of the status of [Crimea].” The resolution draws attention to the obligation of all States and international organizations not to recognize or to imply the recognition of Russia’s annexation.
Source: ‘Annexation of Crimea by the Russian Federation, Wikipedia
According to Wikipedia, 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 a pro-Russian government in Crimea and the declaration of Crimea’s independence. A referendum on access to Russia was held on 16 March despite opposition from the Ukrainian government. The results of referendum have been questioned and the means by which the referendum was conducted were widely criticised by foreign governments and in the Ukrainian and international press. On 17 March, following the official announcement of the referendum results, the Supreme Council of Crimea declared the formal independence of the Republic of Crimea. President Putin officially recognised the Republic of Crimea ‘as a sovereign and independent state’ by decree and approved the admission of Crimea and Sevastopol as federal subjects of Russia. A Treaty on Accession of the Republic of Crimea to Russia was signed between representatives of the Republic of Crimea and the Russian Federation on 18 March 2014 to lay out terms for the immediate admission of the Republic of Crimea and Sevastopol as federal subjects of Russia and part of the Russian Federation. It was ratified by the Federal Assembly by 21 March and almost immediately signed into law by President Putin. Crimea’s admission to the Russian Federation was considered retroactive to 18 March.
Source: ‘Annexation of Crimea by the Russian Federation,’ Wikipedia