The greatest increase in tensions, however, came during the Ukraine crisis that began in 2014, which saw the Crimean peninsula annexed by Russia. Russia also inflamed a separatist uprising in the Donbass region, though Moscow continues to deny its involvement. The United States responded to these events by putting forth sanctions against Russia, and most European countries followed suit, worrying about Russian interference in the affairs of central and Eastern Europe. October 2015 saw Russia, after years of supporting the Syrian government indirectly, directly intervene in the conflict, turning the tide in favor of the Assad regime. Russia's relations with Turkey, already strained over its support for the Assad regime, deteriorated further during this period, especially after the Turkish Air Force shot down a Russian jet fighter on 24 November 2015. In 2015, Russia also formed the Eurasian Economic Union with Armenia, Kazakhstan, and Belarus.