The Turkish Armed Forces launched a long heralded incursion against US-backed Syrian Kurdish forces in northern Syria today, upending a five-year status quo that saw the United States prioritize the campaign against the Islamic State over its relations with its NATO ally Turkey.

Turkish President Recep Tayyip Erdogan announced the news via Twitter, saying the operation, called “Springs of Peace,” was aimed “to prevent the creation of a terror corridor across our southern border.” The planned magnitude of the Turkish assault against the Syrian Kurdish Protection Units (YPG) operating under the umbrella of the Syrian Democratic Forces (SDF) remains unknown.

Initial reports suggested the Turkish assault backed by its Syrian Arab rebel partners called the “National Army” would be limited to an area between Ras al-Ain and the mainly Arab town of Tell Abyad after US forces pulled out of there following orders from President Donald Trump announced late Sunday.

SDF spokesman Mustafa Bali said that Turkish warplanes had attacked the region, creating “huge panic among people.” The SDF’s Coordination and Military Operations Center asserted via Twitter that the bombing was “intensive” and had caused civilian casualties, though it gave no details. Bali threatened Ankara with “a total war on the entire border in order to protect ourselves and our people” and said a general mobilization of the population had begun.