IF ANY international norm can still be called uncontroversial, it is the stricture against cross-border aggression by one sovereign state against another. Certainly any failure to enforce it in one place invites violations elsewhere. That is why Vladi­mir Putin’s decision to send Russian forces openly into Ukraine in the past 48 hours is a watershed, not a mere “continuation of what’s been taking place for months,” as President Obama understated the case Thursday. If Mr. Putin does not pay a high price for this naked, if still cynically denied, attack on his neighbors, the precedent could sow instability far and wide — from the Baltic Sea, ringed by small, free states with large Russian minorities, to the South China Sea, dotted with islands that China covets but other countries claim.

David Fuller's view

I think this is a good editorial.