How did the Syrian Conflict devolve from Arab Spring to outright civil war? What role have outside countries played in the conflict?

The Syrian conflict traces its origin from the Arab Spring, which was the forceful change of governments in different Arab countries such as Tunisia, Egypt, and Libya. The masses were exercising their rights by protesting against what they termed as poor leadership. The street protests escalated to violence between pro-government and anti-government forces.
In Syria, the government used force to fight the dissent expressed by the people. In turn, the people took up arms to defend themselves and fight the government forces. Rebel groups were formed, and the situation deteriorated into a full-blown civil war, claiming hundreds of thousands.
Intervention by the international community has also contributed to fueling the civil war. The United States and its coalition partners have been accused of backing the rebels against the Syrian government under Assad. On the other hand, Russia and Iran have been accused of targeting western-backed rebels and supporting government troops.
http://en.asaninst.org/contents/issue-brief-no-44-syria-and-the-arab-spring-unraveling-the-road-to-syrias-protracted-conflict/

https://www.bbc.com/news/world-middle-east-26116868