Early on, the U.S. looked for cracks in the regime it could exploit to encourage a military coup, but found few.

The efforts reflect how President Barack Obama’s administration has grappled to understand and interact with an opaque Middle East dictatorship run for 45 years by the Assad family.

Unlike the secret White House back channel to Iran, however, the Syria effort never gained momentum and communication was limited. This account is based on interviews with more than two dozen people, including current and former U.S. officials, Arab officials and diplomats. Most of these contacts haven’t been previously reported…

The administration’s core message never strayed from the U.S. line that Mr. Assad ultimately has to step down. But instead of persuading Mr. Assad to exit, the covert communications may have fed his sense of legitimacy and impunity.