Israel. Mainly. Assad, Israel could deal with despite his ties to Iran. Cause Assad's Syria at least was stable and not interested in a war with Israel. But when it looked like the rebels were going to win, Iran threw in Hezbollah fighters from Lebanon (who they back) with experience fighting Israel (and seen as winners the last time by many in the Arab world) who were even more ruthless and efficient killers than Assad's troops. The tide shifted and it looked like Assad would win now with Iran more involved through Hezbollah.
The Sunni powers (Saudis et.al.) thew money to the rebels (to counteract Iran's input) but knew, some think and I am inclined to agree, they needed more ruthless and efficient killers than moderate rebels so jihadists (Al Queda et. al.) with other battleground experience and better armed were thrown in on the rebel side. This stopped the Assad advance. What the US wants is to keep either side from dominating for now until either the moderate rebels can become a unified force that can be backed for taking over if and when Assad falls or can at least keep the jihadists at bay or in the minority.
But the jihadists started turning the tide too much and it looked like they might join forces with the moderates in that Damascus suburb so it looks like Assad was willing to use nerve gas to frighten the moderates out, so his troops could take it over and keep the jihadists from taking control and having that advantage to take over Damascus from. (There are those who say it was the jihadists who had the nerve gas to use against Assad's forces but mishandled it leading to the deaths, or deliberately caused them to make Assad look worse and motivate the US to get involved on their side, but that doesn't seem likely given the way it went down.) If it was Assad and no one does anything, he may feel emboldened to use more nerve gas in entrenched rebel areas and defeat the rebels entirely (and Iran has the experience of Iraqi Sunnis under Saddam using nerve gas against their youthful troops in that war, successfully, and that was when the US was backing Saddam!).
That might leave Hezbollah stronger and willing to try and take over Lebanon, leaving Israel even more exposed and Hezbollah bigger and more threatening to them. It looks like Obama's military and other advisers (cabal to some) want to make it clear to Assad that that won't be tolerated. Some quick strikes, enough to keep Assad busy for a hopefully long time, and afraid to use nerve gas, and the balance to continue until the moderate rebels are better armed and organized so they can control or at least contain the power of the jihadists.
The only way Russia comes into it is their port in Syria they want to maintain and are afraid to lose, plus Syria is a source of income (arms and other stuff) and a buffer for them in the region. But they don't want a military confrontation with the U.S. or Israel for that matter, or Turkey (Turkey is a key player but stuck also cause they don't want a wider war, or their president doesn't, especially after the protests against his policies etc.).
Obama will say at least privately that a strike would also be a warning to Iran which might be emboldened to develop their nuclear bomb if Assad gets away with using nerve gas, but that's a rhetorical argument, because Iran is going ahead anyway in whatever ways it can without starting a war, and I think probably privately Obama doesn't care if Iran gets a nuclear weapon cause it's inevitable, but Israel does and the right in Israel want Iran bombed as well as Syria and anyone else they find threatening.
Most of it is rhetorical, cause no one wants a wider war except maybe the jihadists and Hezbollah and some on the right in Israel and elsewhere. It's kinda like when The Rolling Stones hired the Hell's Angels as crowd controllers at Altamont, only in this case everybody involved is playing with the devil in the hopes they can control him.