The infighting is a new low for an opposition that was never able to unite its military or civilian operations. Across the expanse of the battlefield in Syria, in places like the northeastern province of Raqqa and the divided city of Aleppo, rebels are attacking each other and their supporters with regularity and ferocity. Competition for recruits and weapons — and the right to define the character of the future state — has fueled the interrebel battles.

“The Islamic State wants to eliminate the Free Syrian Army higher command,” Ahmed Farzat, a Free Syrian Army lieutenant, said in an interview on Skype from the central city of Homs, where rebels are struggling against a fierce government assault. “In other words, to marginalize it and replace it.”…

For a time, that success on the battlefield won the support of many oppositions fighters and activists, who are eager to have a powerful ally. But the prospect of victory has receded as government forces have reasserted themselves with the help of Russia, Iran and the Lebanese militant group Hezbollah. And now some rebels and activists find themselves threatened by fighters they once saw as allies.

“The sea is in front of us, and the enemy is behind us,” said Sheik Jassem al-Awad, a tribal leader in Raqqa, adding that he felt squeezed between the government and the radical Islamists. “The Free Syrian Army cannot open two fronts at once.”