8. Proxy warGetty Images
What began as another Arab Spring uprising against an autocratic ruler has mushroomed into a brutal proxy war that has drawn in regional and world powers.
Iran and Russia have propped up the Alawite-led government of President Assad and gradually increased their support.
Tehran is believed to be spending billions of dollars a year to bolster Mr Assad, providing military advisers and subsidised weapons, as well as lines of credit and oil transfers. In September 2015, Russia launched an air campaign against Mr Assad's opponents. Moscow said it was targeting only "all terrorists", above all members of Islamic State, but many of the strikes hit Western-backed rebels and civilians.
The Syrian government has also enjoyed the support of Lebanon's Shia Islamist Hezbollah movement, whose fighters have provided important battlefield support since 2013.
The Sunni-dominated opposition has, meanwhile, attracted varying degrees of support from its main backers - Turkey, Saudi Arabia, Qatar and other Arab states along with the US, UK and France. However, the rise of hardline Islamist rebels and the arrival of jihadists from across the world have led to a marked cooling of Western backing.
US-led coalition aircraft provide significant support to Kurdish militia fighters seeking to defend three autonomous enclaves in the country's north from attacks by IS. But a programme to train and arm 5,000 Syrian rebels to take the fight to IS on the ground has suffered embarrassing setbacks.
"The disappointment caused by the West's inaction created a fertile recruiting ground for extremists, who told those who had lost their loved ones that they were their only hope." - Majed, a 26-year-old civil society activistProduced by Lucy Rodgers, David Gritten, James Offer and Patrick AsareBBC
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