Turkey shot down a Russian fighter jet today after it violated the country’s airspace. The Turkish military claims it issued ten warnings before shooting down the aircraft, while Russia denies any breach of Turkish territory. The incident marks the most serious direct confrontation between a NATO member and Russia in half a century.

Russian President Vladimir Putin responded with vitriol never before leveled at Ankara, accusing it of supporting terrorists in Syria and “stabbing Russia in the back.” The Kremlin’s foreign minister Sergei Lavrov, meanwhile, cancelled his trip to Turkey scheduled for Wednesday and advised Russian nationals to avoid visiting the country.

Bilateral tensions were already brewing. Russian planes violated Turkish airspace twice in early October, in what appeared to be a deliberate challenge to the NATO alliance. Russian aircraft have also been bombing ethnic Turkmen rebels and civilians in the area since last week, prompting Ankara to summon Moscow’s ambassador.

The incident also comes amid U.S.-Turkish negotiations over joint military plans in Syria against the Islamic State (IS). General Paul Selva, vice chairman of the U.S. Joint Chiefs of Staff, is due to visit Turkey today to discuss measures including a possible operation to establish a “terror-free zone” along the Turkish border that would mark a significant expansion of Coalition activity in Syria.

Russian military activity in the region has grown increasingly reckless, and the latest incident marks a tipping point in Russia’s belligerent foreign policy. Despite their NATO obligation to defend Turkey, it is unlikely that the U.S. or other members of the alliance will seek to inflame the situation further. NATO today issued a statement that it stands behind Turkey, and President Barack Obama today stated that Turkey has a right to defend itself. But it’s unclear what this means. Both have repeatedly shown reluctance to respond to Russian hostility – whether in Syria, Turkey, or Ukraine.

Moscow has a range of policy options to respond to the downing of its jet, short of conflict. It has already ordered its citizens not to vacation in Turkey. It could also scrap the Turkish Stream pipeline project or temporarily cut off its natural gas supply. But it is not out of the question that Putin could escalate this crisis. The Kremlin is already deploying more forces to the eastern Mediterranean, which could be used to challenge Coalition access to Syria. Even if Russia did, as it appears, violate Turkish airspace, it will no doubt use today’s incident as a pretext to expand its aggressive behavior in Syria.

Patrick Megahan is a research analyst on military affairs at Foundation for Defense of Democracies, where Merve Tahiroglu is a research associate focused on Turkey. Follow them on Twitter: @PatMegahan and @MerveTahiroglu