DIYARBAKIR, Turkey — Developments in northern Syria, which has a large Kurdish population, have long been on Turkey’s agenda. Turkey considers the growing power of the Kurdish Democratic Union Party (PYD) and the People’s Protection Units (YPG) in the region seriously disturbing.

Turkey has consistently warned that it could intervene militarily in the region. But the prevailing view was that Turkey refrained from military action so as not to confront the United States and Russia. That restraint ended April 25 when Turkish jets bombed YPG facilities at Mount Karachok, near the Syrian town of Derik, and Kurdistan Workers Party (PKK) targets near Iraq’s Sinjar.

The target in Syria was the YPG command headquarters. The YPG said 20 of its militants were killed. The air attacks were followed by small-scale skirmishes on the border. Neither the Kurds nor the allies were expecting such an attack from Turkey. But the real surprise came a few hours later.

After the Turkish airstrikes, the sounds of airplanes were suddenly heard above the region the Kurds call Rojava. The sounds went on for a long time without a break. People in the region at first thought the Turkish planes were back, but a bit later helicopters started to fly over the area — the reality began to emerge later. The planes and helicopters belonged to the anti-Islamic State (IS) international coalition. They crisscrossed the region for several hours, giving the impression of a de facto protection zone. Shortly afterward, action began on the ground. Armored vehicles flying US flags began patrolling the Turkish border. A heavy US armored presence was seen in areas close to Turkey’s key deployment points.