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 this week is a watershed, not a mere “continuation of what’s been taking place for months,” as President Obama understated the case Thursday. If 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.

The reasons for Putin’s escalation, after months of destabilizing Ukraine through more covert means, may be only guessed. Ukraine’s military has made gains against Russian-instigated “separatists” in two key cities, Luhansk and Donetsk, and Putin may have felt that he could not abandon them without incurring political risk in Moscow. The Russian army’s move on Novoazovsk, well to the south of these contested areas, relieved the pressure on them — and perhaps foreshadows seizing a land corridor to Crimea, which Putin absorbed through force and chicanery six months ago but has struggled to resupply by air and sea since. Putin’s strategic goal could be even grander: the takeover of southeastern Ukraine, which he calls “New Russia,” and its incorporation into his ballyhooed Eurasian Union.