With tens of thousands of Turkish troops massed near the border, a high-level Defense Department delegation plans to present what U.S. officials describe as a final offer to address Turkey’s concerns at a meeting Monday in Ankara.

The meeting marks the climax of a years-long dispute between the two NATO allies over U.S. support for Syrian Kurdish fighters who have led the ground war against the Islamic State, but whom Turkey considers a terrorist threat to its own security. Kurdish-led victories against the militant group have effectively left them in control of much of the border area.

Failure of the U.S. effort could throw the war-devastated region into even deeper turmoil, endangering efforts to rout Islamic State remnants and President Trump’s goal of withdrawing U.S. troops from Syria.

The proposal includes a joint U.S.-Turkish military operation to secure a strip south of the Syria-Turkey border that would be about nine miles deep and 87 miles long and from which the Kurdish fighters would be withdrawn.