When Russian President Vladimir Putin set off on his Syria adventure, one big concern was the potential for a disastrous military encounter with the United States.

On Tuesday we got a variation on that theme: Turkey, a NATO ally, shot down a Russian fighter jet that Turkey said had ignored warnings and intruded into Turkish airspace.

The fight against Islamic State in Syria gets even more complicated as a result.

Putin has been freelancing, taking on Islamic State militants and the anti-government rebels who are aligned with the U.S., all to prop up Syrian President Bashar Assad, who controls but a sliver of his country.

The question is whether a furious Putin will be bullheaded enough to attempt retribution on Turkey for the loss of his plane and crew. Or will Putin recognize that meddling over Syria and Turkey has a grave military as well as diplomatic cost?

Turkey said it tracked two Russian jets along the Syrian border and repeatedly warned them off. The planes violated Turkish airspace.

One turned away, one didn’t. Turkish F-16s fired, bringing the second plane down inside Syria. U.S. officials confirmed the incursion, Russia denied it and an angry Putin called Turkey’s action “a stab in the back.”

The Russian jets apparently crossed over a sliver of Turkish territory that juts into northern Syria, violating airspace for just a few seconds.

That means Russia invaded NATO airspace, a provocation that invited the military response.

This was no one-off accidental engagement: Turkey has complained about Russian planes flying over Turkey.

Last month the Turks shot down an unidentified drone. In Europe, where Putin has grabbed Crimea from Ukraine, Russia is testing NATO’s resolve for the first time since the end of the Cold War.

In Syria, Putin is defending his beleaguered ally, Assad, though Assad is guilty of atrocities against his people.

Putin deployed his forces in September, using as cover his proclaimed desire to join the fight against Islamic State. But he trained his weapons primarily on the anti-Assad forces aligned with the U.S. and Turkey.

When Islamic State blew up a Russian jetliner that had flown out of Egypt, and launched the stunning assault that killed 130 people in Paris, everything changed.

Last week Russia and France began cooperating in the bombardment of Islamic State strongholds in Syria. That was a promising development.

Russia has a lot of enemies to keep track of in Syria, including ethnic Turks who are part of the broad anti-Assad coalition and are supported by Turkey.

When the Turks shot down the Russian fighter, the Syrian Turkmen rebels were waiting. They reportedly killed one of the two pilots who ejected from the plane and killed a Russian who was part of a rescue team.

The second pilot survived.

According to The Washington Post, the last time a NATO country shot down a Russian fighter jet was during the Korean War, when U.S. Navy fighters engaged the Soviets in the Sea of Japan.

This is a dangerous moment. Retribution by Putin against a NATO member would be enormously reckless, but he hasn’t been known for caution. This incident should mark the end of his freelancing in Syria.