The rebel war on the so-called Islamic State (ISIS or pejoratively, Daesh) in 2014 ignited an international conflagration whose complexity became almost incomprehensible in 2015. Bashar al-Assad’s fascist regime, anti-regime rebels, Syrian Kurdish People’s Protection Units (YPG), regional sectarian Shia militias aligned with Iran, Al-Qaeda’s Syrian affiliate Jabhat al-Nusra, the Kurdish Workers’ Party, and the governments of Iraq, Iraqi Kurdistan, Jordan, Iran, Turkey, France, the United Kingdom, the U.S., and Russia are all fighting Daesh and many of them are fighting each other.

Despite the incredible complexity of the Syrian theater of this war — a four-sided struggle between the regime, the rebels, ISIS, and the YPG — the war’s evolution throughout 2015 was driven principally by two factors:

The growing military exhaustion of the regime. Escalating military and diplomatic intervention by foreign powers in support of three of the war’s four sides.

In January, as the YPG, Iraqi Kurdish peshmerga, and small groups of Free Syrian Army (FSA) fighters were expelling ISIS from Kobanî with the U.S. acting as the YPG’s air force, Jabhat al-Nusra exploited the Obama administration’s unwillingness to act as the rebels’ air force by launching attacks against the U.S.-backed Steadfastness Movement (Harakat Hazm) in Idlib, Aleppo, and Hama governates. By early March — having proven unable to withstand Jabhat al-Nusra’s assaults without assistance either from fellow rebels or U.S. airstrikes — Harakat Hazm dissolved itself. At the same time, ISIS was driven out of Kobanî city and canton after losing 5,000 fighters during six months of combat against the U.S.-backed YPG.

Harakat Hazm’s destruction was followed by a series of stunning rebel victories over regime forces in Idlib and Hama governates spearheaded by the Army of Conquest (Jaysh al-Fatah), a new Islamist-jihadist alliance consisting of Jabhat al-Nusra, the Islamic Movement of the Free Men of the Levant (Harakat Ahrar ash-Sham al-Islamiyya, or popularly, Ahrar al-Sham), Soldiers of al-Aqsa (Jund al Aqsa), Truth Brigade (Liwa al-Haqq), Army of the Sunnah (Jaysh al-Sunna), Union of the Levant (Ajnad al-Sham), and the Legion of the Levant (Faylaq al-Sham). Army of Conquest-led offensives in March, April, and May systematically cleared checkpoints, villages, and towns the regime held for years in battles lasting days and weeks. Such rapid advances were unprecedented in a protracted war characterized by stalemate. The key to this startling success was unprecedented unity and collaboration between groups participating in the offensive. Each group was assigned responsibility for specific military tasks based on battlefield geography, creating a sophisticated military division of labor and allowing them to utilize combined arms warfare that regime forces have yet to master.

Army of Conquest-led forces typically encircled and assaulted regime positions from all sides simultaneously but left one corridor open for regime forces to retreat through. Lining the exit corridor were FSA groups armed with U.S./Saudi-supplied anti-tank missiles who made every regime retreat a costly affair marked by catastrophic losses of equipment and men.

And Idlib was not the only governate where the regime was losing battles and losing ground.

In Daraa, the FSA’s Southern Front alliance seized Bosra city and its ancient ruins on March 25. Idlib city fell to the Army of Conquest on March 28. The regime lost its last border crossing with Jordan to Jabhat al-Nusra on April 1 (who subsequently turned control of the crossing over to the Southern Front). In mid May, ISIS took advantage of the U.S. refusal to serve as Assad’s air force and crossed the Homs desert unscathed to snatch the city of Palmyra and its UNESCO World Heritage sites from regime control. Less than a week later, dozens of regime soldiers barely escaped with their lives from the rebel encirclement of Jisr al-Shughour National Hospital in Idlib. In June, the Southern Front seized Brigade 52.

A spring of regime defeats gave way to a summer of stalemate and a major failed rebel offensive to seize Daraa city. By contrast, the YPG’s struggle against the fascists of ISIS was anything but a stalemate. Backed by vigorous U.S. airstrikes and robust material support, YPG expunged ISIS from much of the Syrian-Turkish border, territorially united two out of Rojava’s three cantons for the first time, and made steady progress against ISIS in Hasakah governate.

Although the rapid succession of regime losses ended in the first half of 2015, they were decisive in shaping developments in the second half of 2015: on the one hand, they jump-started the stalled diplomatic process but on the other led to ferocious escalation by Assad’s second air force — Russia.

Russia began bombing mostly non-ISIS targets in September. By October, Russian military action started paying diplomatic dividends when the U.S. invited Iran to join international Syria peace talks. (Iran was barred from previous talks over its refusal to sign onto the Geneva communique.) In November, foreign powers backing both the regime and the rebels united to form the International Syria Support Group (ISSG) and issued a statement outlining a schedule and a process to end the regime-rebel war that fueled the rise of ISIS.

The simultaneous intensification of diplomacy and war indicate that the four-sided struggle over Syria’s future is reaching its peak.

Russian airstrikes killed more rebel leaders in four months than the regime was able to kill in four years, including the leader of Army of Islam, Zahran Alloush. States backing the rebels countered Russian escalation by increasing shipments of anti-tank missiles to northern rebels, slowing the pace of regime gains and increasing their cost. Russia in turn began relentlessly bombing the Azaz border crossing — the transit point for these missiles — which led to a sharp decline in their use. With Russian air support, Iranian, foreign Shia, and regime ground forces turned the tide against the rebels, particularly in Latakia and Aleppo governates. This shift in military momentum guarantees that the regime will begin negotiations with the rebels in early 2016 from not only a position of strength but of growing strength.

On the diplomatic front, Saudi Arabia convened a conference to weld together the disparate rebel factions and famously fractious exiled opposition into a single bargaining unit. The aim was not only to create a united, credible political-military force capable of delivering on agreements concluded with the regime but to begin splitting the rebels from the jihadists who will never accept a political settlement no matter what the terms are.

In the unlikely event Russia and Iran are serious about the ISSG’s proposed ceasefire and political process starting in January 2016, 2015 will mark the beginning of the end of the democratic revolution’s armed phase and eventually result in a return to unarmed struggle against the regime, albeit on a higher, more complicated plane than in 2011. If Russia and Iran are not serious, then the Vienna process will end in failure and subsequent mutual military escalation just as Geneva 2 did.

Either way, 2015 heralds the beginning of the end of ISIS whose sole victory on the Syrian battlefield this year at Palmyra depended on the contradiction between the U.S.-led anti-ISIS coalition and the Assad regime, a contradiction that is no longer operative thanks to Russia. The destruction of Daesh’s pseudo-Caliphate in Syria is not a question of if but of when, how, and most importantly, by whom.

Revolutionary War Round-Ups: