It is, in fact, hard to imagine a peace conference that has been convened under less propitious circumstances.

Secretary of State John Kerry announced during a trip to Moscow in May that the United States, along with Russia, wanted to convene the peace conference, an idea first discussed the year before in Geneva. But since that announcement, President Bashar al-Assad of Syria has strengthened his military position, the fractious opposition has become more divided, and Russia and the United States have differed on how to interpret the mandate for the meeting.

The United States’ leverage over the Assad government, meanwhile, has declined. Mr. Kerry arrived at the State Department last year declaring his intention to change Mr. Assad’s “calculation” about his ability to hold on to power. But the Obama administration withdrew the threat of force in return last fall for an agreement that requires Syria to eliminate its chemical arsenal, while the American effort to train and equip Syrian rebels, by all accounts, remained very limited.

“For any political conference to succeed in trying to defuse, much less settle, an intense conflict, the ground has to be laid,” said Dennis B. Ross, a former Middle East envoy. “An agenda needs to be agreed, the parties have to want some minimal achievement, the convening co-sponsors have to share some basic goals, and there has to be sufficient leverage on those doing the fighting to permit some compromises to be made. Most of these conditions are lacking.”

Added a Western diplomat involved in preparations for the talks: “We don’t have a Plan B.”

Unlike the Middle East talks, in which Mr. Kerry set a nine-month goal for completing a peace treaty, there is no target date for completing the Syria peace talks or establishing a transitional administration that could take over if Mr. Assad agreed to relinquish power. In a closed-door meeting with the Syrian opposition last year, Mr. Kerry noted that Vietnam peace negotiations had gone on for years.