OK, I'll try to lay this out the best I can, bearing in mind that some may disagree.
To me, a war in Syria authorized by congress is legal. That's probably not in dispute. Stupid? Sure. Illegal? Nope.
A war denied by congress, but pursued anyway, is not legal in my mind. As I said previously, I'd probably obey anyway (while writing my representatives asking them to start impeachment proceedings.) This has nothing to do with the legality of the order, and everything to do with my troops. Also, the folks protesting in uniform? I will no longer have an objection to that if congress says "no" and Obama goes anyway. Such an order will be , in my mind, illegal and unconstitutional, and I cannot fault a man for standing up against that, even if I won't myself.
An order to use military force against American citizens in any of the scenarios we are warned about in fiction, I would not obey. Even if authorized by congress, mass confiscations of weapons, suppression of dissent, etc etc.... those things are all unconstitutional. Again, it's like simon says. Congress and the president can give all the orders they wish, but if they're unconstitutional, I have not only the RIGHT to deny, but the DUTY.
Some may ask why the disparity. Why would I obey an unconstitutional order to invade Syria after denied by congress, but refuse an order (even duly authorized by congress) to, for example, collect all weapons from private citizens and melt em down, round up <insert group here> and put them in camps, etc?
Quite simply, my loyalty to my troops ends where my loyalty to the American public begins. Although I love my troops, they know that if there's ever an occasion where the US military is called to oppress citizens and violate their human rights, take their weapons, etc... then they better hope they don't see me again, because it'll be them looking down my rifle barrel.
I'm willing to sacrifice my duty to the constitution to go invade Syria if necessary, because I care about my troops, and they need me. I have no loyalty to Syrians on either side, and to me conducting a war there is a small price to pay to take care of my troops.
I am not, and never will be willing to sacrifice my duty to the constitution in order to conduct a war against Americans, gun owners, jews, christians, or whatever group they want me to stomp under a jackboot. My troops will either come with me, or they'll become my enemy. Authorized by congress or not, such a campaign renders my committment to the country null and void, IMHO.
I DO realize that this makes me a hypocrit. However, it's a hypocrisy I can live with.