This is both a "casus belli" (Act of War) and a "casus foederis" (Act of War towards an ally; in this case Germany).
Not to mention, this Act of War was unilaterally committed on behalf of the US people WITHOUT the Constitutionally necessary decree from Congress.
According to the UN charter, Russia would be well within their rights to declare this a just war ("jus ad bellum") and take necessary defensive & retaliatory action against the US and her allies. World War 3 has already begun and most people don't even know it yet.