In December 2011, when car bombs began hitting government security buildings — and killing civilians nearby — government supporters and opponents alike viewed the explosions as an ominous turn in the conflict.

Until then, the fighting had largely pitted rebels with small arms and roadside bombs against the army and security forces. But suddenly, the Syrian capital was witnessing scenes reminiscent of the Iraqi insurgency. Checkpoints and blast walls went up everywhere.

Some in the opposition said they suspected the government of setting the bombs to tarnish the uprising. But one rebel group, the extremist Nusra Front, began claiming responsibility for many of those attacks. That led to one of the first signs of the split in the armed opposition, between those who said they were defending themselves against a violent government crackdown and a minority who called for an Islamic state. And it repulsed some civilian activists who then distanced themselves from the movement.

Now, the Nusra Front has become a major force on the battlefield, leading other rebel groups in more conventional fights. That poses a quandary for the United States, which supports the opposition but rejects the Nusra Front and accuses it of ties to Al Qaeda.

The bombs have killed Syrians of all sects and views. In November, a car bomb hit the Damascus neighborhood of Mezze 86. News media speculated that the neighborhood had been chosen because it includes military families and members of Mr. Assad’s minority Alawite sect.

That characterization infuriated Salah, a graphic designer who lives there. Among the dead, he said in an e-mail interview, were his wife, Amal, and 7-year-old daughter, Hanan.