ALEPPO, Syria — The Islamic State (IS), arguably the most dangerous and brutal terrorist group in modern history, now controls more than a third of Syria, according to the Syrian Observatory for Human Rights, a prominent opposition nongovernmental organization. The group’s stunning victories in Iraq have been repeated in Syria, though to a lesser extent and to even less media attention. IS now controls nearly all of the oil-rich eastern province of Deir ez-Zor, with regime forces controlling only a few pockets. Crucially, after ousting its rival, the al-Qaeda-affiliated Jabhat al-Nusra, from the provincial capital, IS is now face to face with regime forces, the two sitting opposite one another on the front lines that divide this largely devastated city.

The undeclared truce between the regime and IS seems to have run its course after both sides achieved all they could in their marriage of convenience. Nothing better illustrates this than the devastating assault launched by IS on the al-Shaer gas field in Homs on July 16 that may have left up to 300 regime troops and civilian employees dead. Meanwhile, the regime continues to launch airstrikes on IS headquarters and training camps in Raqqa, an indication that the gloves are now definitely off.

As IS consolidated its grip on Deir ez-Zor, after a humiliating rout of Jabhat al-Nusra that saw its leader, Safwan al-Hant, captured and killed July 14 while trying to escape disguised as a woman — his own men having reportedly turned him over after defecting — it quickly turned its expansionist intentions elsewhere. IS has also resurfaced in rebel-controlled areas near Damascus, including the Yarmouk Palestinian refugee camp, and threatens to derail fragile and painstakingly negotiated local cease-fires and agreements.

It is also launching its own campaign in Ghouta against the Islamic Front, headed by the Saudi-backed Jaish al-Islam, whose leader, Zahran Alloush, said that fighting IS is more important than fighting the regime.