BEIRUT (AP) — Syria's civil war has been one of the modern world's most brutal conflicts and one of its most heavily filmed. Hundreds of thousands of amateur videos uploaded to YouTube document every heartbeat of the war over the past seven years, from momentous events like cities under bombardment to intimate scenes like a father cradling his dead children.

Syrian activists fear all that history could be erased as YouTube moves to rein in violent content. In the past few months, the online video giant has implemented new policies to remove material considered graphic or supporting terrorism,...