Turkish officials have kept a strong grip on the communities along the border, attending funerals while laying the blame for the casualties on the United States for supporting what Ankara calls a terrorist organization. Ottoman-era martial music played repeatedly on loud speakers in the border towns, and the mosques recited prayers for the Turkish army.

But despite the government’s insistence that it is fighting terrorism to protect Turks, President Recep Tayyip Erdogan’s campaign against the Kurdish militia in Syria has hurt communities at home, with already 20 dead and 80 wounded in Turkey.

It has also re opened old wounds and anxieties in southeast Turkey’s deeply traumatized population . Syrians are reliving the horrors of war. For the Kurds, many of whom distrust the intentions of the central government in Ankara , it is only reinforcing longstanding disaffection.

Young people gathered at the scene of the mortar strike in Nusaybin were even questioning whether the mortars had been fired on the town by Turkish forces, although the trajectory indicated they had been launched from Syria.

Those skeptical of the official account noted that throughout the course of Syria’s long civil war, the Syrian Kurdish group had never so much as thrown a single stone across the border, contradicting the government’s talk of a terrorist threat.

While these deadly mortar attacks are new, Turkey’s southeast is no stranger to conflict.

The majority-Kurdish population has lived through decades of violence as the Kurdistan Workers’ Party, or P.K.K., has waged a separatist insurgency in Turkey. Thousands have been killed in Turkish military campaigns against the P.K.K., and tens of thousands of civilians have been displaced since the 1980s.