The second would combine shelling with cross-border raids to target Kurdish militants in Syria. Turkey’s policy, after all, is not just about Syria. It also depends on the Kurdistan Workers’ Party, known as the P.K.K., and its Syrian affiliate, the Party for Democratic Unity, or P.Y.D. Turkey views the P.K.K. as an existential threat, and the P.Y.D. is reportedly already active in Syrian towns near the Turkish border, though the group has said it does not plan to fight Turkey. If Turkey believes that Kurdish militants are turning Syria into a staging ground for operations against Turkey, the Turkish military would strike decisively, as it did against Kurds in northern Iraq after Saddam Hussein’s rule effectively ended there in the 1990s. Ankara might go for the “northern Iraq option” once again to prevent Kurdish militants from taking control of northern Syria.

Finally, if things get worse along the border, causing more Turkish casualties, Turkey may go even further, staging a limited invasion to contain the crisis as it did in Cyprus in the 1970s. At that time, Ankara waited patiently for the United States and the international community to come to its aid in Cyprus. When such help did not materialize, Turkey took matters into its own hands, and landed troops on the island.

NATO has already issued a statement that it will defend Turkey against Syria. Yet if Turkey decides that the international community is not going to actually help stave off the Assad regime’s aggression, it may choose the Cyprus option. The Turkish president, Abdullah Gul, has suggested that Ankara may be getting closer to its threshold, declaring on Oct. 8 that “worse-case scenarios” are looming in Syria and calling upon the international community to act.

The Cyprus scenario is the least desirable for Turkey. Full-scale war is not in its interest, especially if Turkey launches such a campaign without American backing. And NATO support under Article 5 of the alliance’s charter, which calls for all NATO members to come to the defense of any member that is attacked, would be harder to muster. Article 5 has been activated only once in NATO’s history and that was after the 9/11 attacks. Moreover, European nations like France haven’t in the past been keen to come to Turkey’s defense. A unilateral war against the Assad regime would also irritate the United States and anger Russia and Iran, Ankara’s rivals in Syria — a serious concern because Moscow and Tehran have a track record of supporting Kurdish militants.

The northern Iraq option would not necessarily raise America’s ire, but it would expose Turkey to further P.K.K. attacks, including ones backed by Iran. Tehran already appears to be encouraging the P.K.K. to punish Turkey for its stance in Syria. Major attacks could hurt Turkey’s economy and erode Mr. Erdogan’s popularity.