A Russian Su-24 warplane was shot down Tuesday by Turkey near the Turkish-Syria border. Turkey claimed responsibility for the downing, saying it had violated Turkish airspace and the move was "within engagement rules." Russia insisted the plane was in Syrian airspace on a mission to strike the Islamic State. Russian President Vladimir Putin called the downing "a stab in the back delivered by accomplices of the terrorists."

This could be one of the most critical moments since the end of the Cold War. Its potential severity and associated risks could be a climax of geopolitical crises over the past decades. This is the first time that Russia has suffered such big losses.

Putin is faced with a choice that carries more risk than when he considered whether or not to annex Crimea. After calling Turkey's move "a stab in the back" for Russia, he apparently would be under more pressure to make Turkey pay a price for the downing and his personal prestige and Russia's tough image will be tested.

Moscow can retaliate against Turkey in many ways, but this means it will face a Cold War-style confrontation with NATO. Turkey didn't seem to rush to this decision. Russian warplanes have entered Turkish airspace at least twice before, which Moscow admitted and sent delegations to consult with the Turkish side. Ankara has repeated that it has the right to shoot down warplanes that infringe on Turkey's territory. Obviously it had the plan and had communicated with NATO.

Ankara must be well aware of what it means to shoot down a Russian warplane. Next it will be careful enough not to give Russia a chance to down one of its warplanes in retaliation. Moscow probably has to cross the Syrian-Turkish border to implement retaliation, which however risks escalating military confrontation.

If Russia strikes down a Turkish warplane in Turkish airspace, or strikes a Turkish air base, it will touch NATO's nerve. If NATO takes no action, its pledge to protect smaller alliance members will be discredited. However, if NATO adopts substantial action toward Russia, Europe will confront an unprecedented turbulent situation not seen since World War II.

We can sense the antagonism from the incident as NATO's eastward expansion triggered Russian countermeasures while Russia's strategic reaction prompts NATO to incrementally intend to constrain Russia.  The US and Russia are clearly aware that their every reaction will send signals about the future order of Europe and the Middle East.

Currently the most burning issue in the world is to counter terrorism. But the overshadowing of geopolitics is too expansive to shun. It remains to be seen whether a rational manner can be adopted by Russia and NATO in the aftermath of the plane downing. We hope peace can prevail.

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