How did Syria develop into a proxy war?

The USA, Russia, Iran, Turkey, Arab Gulf States, and various local and foreign militia are all part of the war in Syria. Why does the conflict have so many competing sides? Guests Selim Atalay, Turkish journalist Adam Ereli, Former US ambassador to Bahrain Bassam Barabandi, Former Syrian diplomat and a co-founder of 'People Demand Change Viacheslav Matuzov, Former Russian diplomat