OK, now the theories for why Syria spiraled so wildly. The first is what you might call "sectarian re-balancing" or "the (CNN’s foreign affairs show host) Fareed Zakaria case" for why Syria is imploding (he didn’t invent this argument but is a major proponent). Syria has artificial borders that were created by European colonial powers, forcing together an amalgam of diverse religious and ethnic groups.
Zakaria’s argument is that what we’re seeing in Syria is in some ways the inevitable re-balancing of power along ethnic and religious lines. He compares it to the sectarian bloodbath in Iraq after the United States toppled Saddam Hussein, after which a long-oppressed majority retook power from, and violently punished, the former minority rulers. Most Syrians are Sunni Arabs, but the country is run by members of a minority sect known as Alawites (they’re ethnic Arab but follow a smaller branch of Islam). The Alawite government rules through a repressive dictatorship and gives Alawites special privileges, which makes some Sunnis and other groups hate Alawites in general, which in turn makes Alawites fear that they’ll be slaughtered en masse if Assad loses the war. (There are other minorities as well, such as ethnic Kurds and Christian Arabs.)
The second big theory is a bit simpler: that the Assad regime was not a sustainable enterprise and it’s clawing desperately on its way down. Most countries have some kind of self-sustaining political order, and it looked for a long time like Syria was held together by a cruel and repressive but basically stable dictatorship. But maybe it wasn’t stable; maybe it was built on quicksand. Bashar Assad’s father Hafez seized power in a coup in 1970 after two decades of extreme political instability. His government was a product of Cold War meddling and a kind of Arab political identity crisis that was sweeping the region. But he picked the losing sides of both: the Soviet Union was his patron and he followed a hard-line anti-Western nationalist ideology that’s now mostly defunct. The Cold War is long-over and most of the region long ago made peace with Israel and the United States; the Assad regime’s once-solid ideological and geopolitical identity is hopelessly outdated. But Bashar Assad, who took power in 2000 when his father died, never bothered to update it. So when things started going belly-up two years ago, he didn’t have much to fall back on except for his ability to kill people.
What about Russia and Iran?
Russia is Syria’s most important ally. Moscow blocks the United Nations Security Council from passing anything that might hurt the Assad regime, which is why the U.S. has to go around the U.N. if it wants to do anything. Russia sends lots of weapons to Syria that make it easier for Assad to keep killing civilians and will make it much harder if the outside world ever wants to intervene.
The four big reasons that Russia wants to protect Assad, the importance of which vary depending on who you ask, are: (1) Russia has a naval installation in Syria, which is strategically important and Russia’s last foreign military base outside of the former Soviet Union; (2) Russia still has a bit of a Cold War mentality, as well as a touch of national insecurity, which makes it care very much about maintaining one of its last military alliances; (3) Russia also hates the idea of "international intervention" against countries like Syria because it sees this as Cold War-style Western imperialism and ultimately a threat to Russia; (4) Syria buys a lot of Russian military exports and Russia needs the money.