Who is threatening their sovereignty?
Putin follows the hardline Russian Nationalist school of thought, which holds Russia is entitled to be sovereign over the lands of Rus, the vast captive lands held by the Rus inside the present boundaries of the Russian Federation, all lands ever held by the Russian Empire, all lands ever held by the Soviet Union, all lands ever seriously coveted by the Russian Empire (i.e., the Dardanelles), and all lands that border territory currently held by Russia.
One will note that the last facet of this ideology means that, by definition, a land border means that Russian sovereignty is threatened. Which bodes ill for the Putin Fanbois position that, if only we would allow the Russian Army to rape, loot, and murder its way across (checks notes), Ukraine, Estonia, Latvia, Lithuania, Poland, Finland, Moldova, Transnistria, Georgia, Azerbaijan, and Nakhchivan, all would be well.