First phase: Clearing of Tahrir al-Sham

The Lebanese group Hezbollah, the Lebanese Armed Forces, and the Syrian Arab Army launched a major operation on 21 July 2017, to destroy the pockets of HTS and ISIL fighters on the Lebanon-Syria border.^[24] During the offensive, the Lebanese Army assumed a defensive position in Arsal.^[25] By the following day, Hezbollah reportedly captured key points near the border, including the strategic hilltop of Dhahr al-Huwa, a former Tahrir al-Sham (al-Nusra Front) base.^[26] As of 23 July, the Syrian Army captured 36 square miles of terrain on the Syrian side of the border.^[27]

On 27 July, a three-day ceasefire agreement was reached by Hezbollah with Tahrir al-Sham and Saraya Ahl al-Sham in the Lebanese portion of the Qalamoun Mountains. The agreement called for Tahrir al-Sham forces to withdraw from Lebanon to Idlib, Saraya Ahl al-Sham forces to withdraw to the eastern Qalamoun Mountains, where rebel forces maintain a pocket of control, and exchanges of prisoners from both sides.^[28] On the same day, Hezbollah announced that there would be further military operations against the ISIL pocket.^[29]

Second phase: Offensive against ISIL

On 4 August, Hezbollah commander Hasan Nasrallah announced that the Syrian Arab Army and Hezbollah would participate in the recapture of the ISIL pocket on the Syrian-Lebanese border for the first time since 2013.^[29] The Lebanese Army announced that it will not coordinate with the Syrian Army during the offensive.^[21] Two days later, the Lebanese Armed Forces launched an assault on ISIL positions near the Shabeb Canyon. After a series of intense clashes, the Lebanese Army captured the sites and pushed on to the Abu 'Ali and al-Dalel al-'Aqra hills in the Ras Ba'albak region. The hills were also eventually seized.^[30] Further Lebanese Army advance was reported on the countryside of Jaroud Arsal and Ras Balbak involving mechanised forces.^[31]

In the morning of 19 August, the Lebanese Army renewed its offensive against ISIL in the Lebanese portion of the Qalamoun Mountains, while the Syrian Army and Hezbollah launched an offensive in the Syrian side of the border.^[32] Later in the day Syrian Arab Army's Republican Guard and Hezbollah captured Abu Khadeir and Mass’oud valleys, Sha’abat Srour, Qabr Al-‘Arsali, Khirbat Al-‘Aylat, Abu Khadeij Point, and Shalouf Point alongside the border with Lebanon.^[33]

On 25 August, around 100 ISIL fighters surrendered to Hezbollah on the Syrian side of the western Qalamoun Mountains. By the same day, ISIL held only 40 square kilometres of territory on the border.^[20] On 27 August, the Lebanese Army announced a ceasefire in order to negotiate over 9 Lebanese soldiers who were captured by ISIL during the Battle of Arsal in August 2014.^[34]

On 27 August, the remaining ISIL holdouts in the western Qalamoun agreed to the ceasefire with the Lebanese Army in Lebanon and Hezbollah and the Syrian Army on the Syrian side of the border. On the next day, ISIL fighters burned their headquarters in the area, and prepared to be transferred to Abu Kamal. This was the first time that such a large group of ISIL fighters had agreed to a surrender deal.^[35]