From the end of February 2014, demonstrations by pro-Russian and anti-government groups took place in major cities across the eastern and southern regions of Ukraine, in the aftermath of the Euromaidan movement and the 2014 Ukrainian revolution. During the first stage of the unrest, known as the "Russian Spring" (), the Ukrainian territory of Crimea was annexed by the Russian Federation after a Russian military intervention, and an internationally criticized (&lt;a href="United%20Nations%20General%20Assembly%20Resolution%2068/262"&gt;based on UN resolution 68/262&lt;/a&gt;) Crimean referendum. Protests in Donetsk and Luhansk regions (oblasts) escalated into an armed pro-Russian separatist insurgency. From late 2014, cities outside of the Donbas combat zone, such as Kharkiv, Odessa, Kyiv and Mariupol, were struck by bombings that targeted pro-Ukrainian unity organizations. To maintain control over southern and eastern Ukraine, the government launched an "Anti-Terrorist Operation" (ATO), sending in the armed forces to quell the unrest.