BEIRUT, Lebanon — Russia, Iran and Turkey met in Moscow on Tuesday to work toward a political accord to end Syria’s nearly six-year war, leaving the United States on the sidelines as the countries sought to drive the conflict in ways that serve their interests.

Secretary of State John Kerry was not invited. Nor was the United Nations consulted.

With pro-government forces having made critical gains on the ground, the new alignment and the absence of any Western powers at the table all but guarantee that President Bashar al-Assad will continue to rule Syria under any resulting agreement, despite President Obama’s declaration more than five years ago that Mr. Assad had lost legitimacy and had to be removed.

Mr. Obama’s reluctance to back that demand with more involvement as the war escalated leaves Washington with little leverage on a geopolitical crisis as President-elect Donald J. Trump prepares to take office.

Mr. Trump’s only recent statement on Syria came last week, when he declared at a Pennsylvania rally that the situation was “so sad” and promised, “We’re going to help people.” He vowed to extract funds from Persian Gulf nations to build “safe zones” in Syria “so people will have a chance,” without addressing the question of who would enforce those zones on the ground or in the air.