Earlier in the three-year-old Syrian uprising, Mr. Assad decided to mostly avoid fighting the Islamic State to enable it to cannibalize the more secular rebel group supported by the West, the Free Syrian Army, said Izzat Shahbandar, an Assad ally and former Iraqi lawmaker who was Baghdad’s liaison to Damascus. The goal, he said, was to force the world to choose between the regime and extremists.

“When the Syrian army is not fighting the Islamic State, this makes the group stronger,” said Mr. Shahbandar, a close aide to former Iraqi Prime Minister Nouri al-Maliki, who said Mr. Assad described the strategy to him personally during a visit in May to Damascus. “And sometimes, the army gives them a safe path to allow the Islamic State to attack the FSA and seize their weapons.”

“It’s a strategy to eliminate the FSA and have the two main players face each other in Syria: Assad and the Islamic State,” said Mr. Shahbandar. “And now [Damascus] is asking the world to help, and the world can’t say no.”