The war in Syria began because the Assad regime could not tolerate a peaceful demonstration which in a solidly democratic nation would have run its course, ended quietly and, hopefully, its complaints taken seriously by the government. Many Syrians were complaining about high unemployment, corruption and a lack of political freedom. the government used deadly force to crush the dissent.
Violence rapidly escalated and the country descended into civil war. Many groups and countries found it useful to intervene because of their own agendas. Some fostered hatred between Syria's Sunni Muslim majority against the president's Shia Alawite sect. Jihadist sects were allowed to flourish. Russia and IRan have supported President Assad so that he has almost regained control.
The US, UK, France and other Western countries have supported what they consider "moderate" rebels. Who is to say what would have happened if no outside country had become involved. Russia is now wanting a separate political negotiating process from that recommended by the UN.