Serious question! And here’s my serious answer:
the damage inflicted by the aggressor on the nation or community of nations must be lasting, grave, and certain
Which aggressor – the Syrian government or the rebels? They’re both aggressing. Why can’t they both lose? Either way, check.
all other means of putting an end to it must have been shown to be impractical or ineffective
and what ways have been attempted? No check here, seems to me.
there must be serious prospects of success
Do we even have an objective by which to measure success? Don’t see a check here, either…
the use of arms must not produce evils and disorders graver than the evil to be eliminated.
The power as well as the precision of modern means of destruction weighs very heavily in evaluating this condition.
If the US gets involved, the best we can hope for is for Assad to say, oh, my, I won’t use chemical weapons again. I’ll just keep blowing my citizens up conventionally! Meanwhile, our getting involved is going to produce all kinds of evils and graver disorders when everyone who hates us comes running over to join the party. I don’t see a check here, sorry.
Pray for peace, like Pope Francis asks us to.