"...Here's my primary concern: Russia, China and Iran have told us to stay out of it..."
Nuclear warfare should be everyone's concern, but nobody (including the Russians and Chinese, nor we) are going to Go Nuclear over some pissant flashpoint like Syria...
And neither the Russians nor the Chinese can project Conventional Military Muscle like we can...
The Chinese aren't as heavily vested in Syria as the Russians, and they're (1) too far away and (2) lack the logistics assets to deploy on the scale required and (3) are far too pragmatic to Kill the Goose That Layed the Golden Egg... they're too deeply intertwined with our economy to make them want to rumble with us except in the most dire circumstances, and pissant Syria isn't that 'dire' to them, in comparison with Pacific Rim and Southeast and East Asia issues...
The Russians are heavily vested in Syria and they are still a formidable nuclear power but they're still chipping rust off their fleet and building new assets to replace the rusted-over ones that have been lying dormant since the Fall of the Soviet Union in the 1989-1991 timeframe... bottom line, the Russians just can't project Conventional Power like they used to, and, if they were going to jump on us, conventionally, they would have done it in Europe before The Fall, rather than with a military that is merely a pale copy of its former glory days as the muscle of the Soviet Union... they're still a force to be reckoned with, regionally, on a conventional basis, but they aren't anywhere near as 'scary' as they used to be, nor will they be, for quite some time, despite Vlad's desperate efforts to catch-em-back-up again...
OUR conventional military, on the other hand, has been kept in fighting trim and well-staffed with combat veterans and other folks chock-full of real-world experience...
The Iranians, on the other hand, continue to be something of a joke, beyond the realm of regionally-capable ballistic missile technology, and all we'd need to do is sic the Israelis on 'em, to make 'em pee their pants. No, I think they'd like for all of this go away quietly, or to contribute as little as possible, to keep 'em out of trouble, and to give 'em the peace they need to continue to build upon their Revolution...
But... all that being said... and assuming, for the moment, that that snapshot summary was more right than wrong...
Just because we CAN do a thing, does not mean that we SHOULD do a thing...
At present, my own feelings about this are...
Let 'em rot... let 'em go on killing each other... let 'em have a field day... let the Euros handle it for once... it's their backyard.