With airstrikes and artillery fire, Turkey defied US appeals and opened a long-anticipated offensive on Afrin, an enclave in Syria for Kurdish militias backed by the US.
Turkish officials have framed the offensive as part of a wider battle against Kurdish separatists, known as the Kurdistan Workers' Party, in Turkey's southwest. Turkey also fears any gains in strength by the Syrian Kurds, whose territory runs along some of Turkey's southern border.
But the US has opted to back the Syrian Kurds as proxy fighters against Isis (Islamic State) and as a buffer to keep the militants from trying to reclaim territory.
The military action immediately raised concerns that it could spark conflicts among the assortment of foreign military powers present, in proximity, across northern Syria. They include Turkey, Russia and the US. All have Isis as a common foe, but, individually, they back different factions among the various armed groups in Syria.
The latest flash point also highlighted the shifting disputes and conflicting agendas that have complicated any efforts towards ending nearly seven years of conflict in Syria. The Turkish military action came amid intensifying violence in the northern Syrian province of Idlib, where Syrian government forces are on the offensive against al-Qaeda-aligned rebels in the east of the province.