Just wrote this on a different forum as a response to a very convoluted description of the same topics and decided to copy it here.

"Boy this was one complicated explanation of something relatively simple! Here is my take on things:

Assad is Shia. In Syria (as well as in Islam) Shia are the minority. Majority are the Sunni. Since times immemorial Sunni and Shia hate each other no less then they hate "the Crusaders", i.e. majority of those posting into this thread.

Since Assad's dadda, Assad Sr., assumed power back in the day, not only did he become a Shia ruling over Sunni in a majority Sunni state, he also took some serious steps to secularize the country, which bought him little love from country's religious crowd in general, while among Sunnis simply placed him on the level of something resembling our notion of "antichrist". Since that day Syria's Sunnis were united by one all-superseding beautiful dream: "to cut Assads' throats" and put some Sunni in power so they could do to Shia what Shia is currently able to do to Sunni.

When events in Tunisia (called Arab "spring" which now, agree, sounds a bit odd not to say sadistic) finally dominoed into Syria, Syrian Sunnis saw their opportunity and rebelled.
Since Assads were long-time Russia's allies Obama administration naturally supported the rebellion, and that brought Syria to a proxy struggle between America and Russia which is continuing today. If Obama administration didn't support Syrian Sunnis, Assad would've easily crushed the rebellion at its inception and by now all would've been long back if not to love then at least to peace for sure.
Btw, ISIS is a Sunni organization whose immediate main objectives are amply implied by its name, ISIS stands for "Islamic State of Iraq and Syria", which is to topple secular Shia governments of Iraq and Syria and replace them with something resembling Saudi Arabia but with bit more of a religious zeal.

For those who are interested: the difference between Shia and Sunni in practical terms can be observed by comparing one of several traditionally structured Sunni states Saudi Arabia to the only traditionally structured Shia state Iran.

In Saudi Arabia King serves as both religious and administrative leader of the State. He does appoint Mufti to oversee day-to-day matters of the religious side of things but can overrule him if he so chooses. The same goes for ISIS where Abu Bakr al-Baghdadi has been designated as the caliph, i.e. the king, and is both administrative and religious leader of the organization.

In Iran those two positions are set up in exactly opposite way: the Ayatollah is the land's religious leader while the president is its VP Operations. Why VP? Because in traditional Shia state religious leader should always be able to overrule the country's administrator. Ayatollah is elected by Council of Experts which for our "gyaur" purposes let's call it a "Council of Ayatollahs", while president is elected democratically by the nation's populace.

What does it all remind you of? R-r-r-r-right! The Catholics (Shia) vs. Protestants or rather Anglicans (Sunni). Shia has Ayatollah (the Pope) while Saudis (and the like) have their own Henry VIIIs!

That's why Iran so vehemently supports other Shia governments in Syria and Iraq despite the later two being decidedly secular.

Turkey on the other hand find themselves in a bit more uncertain and difficult position. On one hand Turks, especially now with more religiously oriented Erdoğan naturally hate Shia Assad and would love to support fellow Sunnis. On the other hand they realize that fellow Sunnis they are facing across the border are either wacko "moderate" rebels that are anything but moderate or ISIS that want Turkey to go 2-500 years back in time. Turkey's main concern is that America gives Iraqi Kurds their independence and that would create huge issues among Kurds in Turkey. All that forces Turkey to be reactive rather then proactive. I kind of feel for them in that regard."