Austria has expelled four Russian diplomats, a move that has triggered a swift and pointed reaction from Moscow. The Russian side announced that its response to the expulsions is inevitable, though it withheld any details about what measures it is preparing. Neither government has named the diplomats or spelled out the official grounds for the order. Expulsions of this kind normally mean individuals are declared persona non grata and given a short window to leave the country. The decision lands at a time when relations between Vienna and Moscow are already strained by the war in Ukraine. Austria's action shows it continues to take the harder line that much of Europe has adopted toward Russian diplomatic staff. Moscow's promised retaliation now hangs over whatever diplomatic exchanges come next.