The timing appeared to be both calculated and opportunistic.

As soon as a deal to end fighting between Israel and Hezbollah in Lebanon was signed last week, another conflict kicked off not far away in Syria.

Syrian rebels launched sweeping assaults against the forces of President Bashar al-Assad in northwestern Syria, taking control of large portions of territory, including much of the city of Aleppo.

The rebels also forced Russian forces to abandon a base near Aleppo and militias backed by Iran to cede control of the town of Tel Rifaat, a Syrian outpost near the Turkish border, according to Syrian humanitarian and research groups.

The sudden advance of the rebels, and the setback for Mr. al-Assad and his allies, has stoked a cauldron of geopolitical rivalries that has simmered in Syria for more than a decade after a democracy uprising in 2011 turned into full-scale rebellion.

It has also underlined how easy it is for violence to spread like wildfire across a volatile region made all the more unpredictable by the intertwining and competing interests of numerous large powers vying for influence.

Iran and Russia, hoping to prop up a key ally in the region, have been providing vital military support to Mr. al-Assad’s government for years. Russian planes have bombed rebel positions, while on the ground, Iranian-backed militias like the Lebanon-based Hezbollah have battled rebel fighters in support of the Syrian government.