WASHINGTON (AP) — A critical conference is scheduled this weekend in Vienna where diplomats will try to decide which of Syria's fighting forces are common enemies and which can be included in a transition government with President Bashar Assad.

The United States, Russia, Saudi Arabia and Iran haven't agreed on much throughout Syria's bloody four-and-a-half year-old civil war. But failure to reach an agreement could leave international peace efforts in tatters.

Diplomats are grappling with questions that have scuttled all previous attempts to forge a ceasefire and usher in a political transition. The main issue is who is a friend and who, other than the Islamic State group, is a foe.

More than 250,000 people have been killed in the Syrian war. Eleven million have been uprooted from their homes.

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