I'm guessing it's because Putin's war

threatens in a very real sense to expand into members of NATO, who will then be forced to invoke Article 5 of NATO's founding treaty for collective defense.  Biden has just declared that American commitment to Article 5 is "sacrosanct".  In other words, should any member of NATO be dragged into this war, the result will be a direct faceoff (rather than a proxy war) between Russia and America and 77 years of world peace will give way to WWIII.