There was a point where in the Syrian Civil War where large scale intervention, including military assistance with US assets was feasible. But now the FSA, the Syrians we could've sided with and actually gotten a democratic, somewhat western friendly nation out of have either been killed off or reduced in power.
Well, first of all, aid in the form of CIA training camps in both Jordan and Turkey as well as small arms and intell was there from the beginning, and has not wavered, Obama is asking for money to continue this. Secondly, as to overt military intervention, you should recall that Obama tried that as well. Having dispatched Hillary Clinton on three occasions to the UN to secure a resolution for the use of force in Syria. All three times she returned empty handed. This of course was due to both China and Russia vetoing all requests. And they did so because of the US/West having abused the resolution for use of force in Libya. The use of force in Libya was authorised to protect civilians, not regime change, which the Russians criticised from the start of the air strikes stating that the targeting was consistent with regime change, and not the protection of civilians!!
In addition to Obama failing to secure UN authorisation for the use of force in Syria, the UK pulled its support for military intervention, Obama couldn't get congressional approval at home, and!! 70% of Americans were against military intervention in Syria! So...........I suppose Obama could have been a belligerent and ordered our military in anyway, I'm sure the hawks would have loved that, but cooler heads did prevail in that instance at least.
In addition to all that, president Assad was the stabilising (albeit imperfect of course) force in Syria that Saddam Hussein was in Iraq that Gaddafi was in Libya. We have managed to kick the legs out from under all three of those countries, and all three of them are in serious trouble. Had president Assad been left alone at the beginning, he would have crushed the rebellion in its infancy and we wouldn't be talking about this. Instead, the US fanned the flames, provided early support breathing life enough into the insurgency until recruits could arrive from neighbouring countries and in due time president Assad's own war on terror has strung out over three years with 150,000 dead civilians as a result.
China and Russia, for their part, both correctly predicted that US interference in Syria would cause that conflict to spread throughout the region, which is precisely what we are seeing today.