Syria

I'm trying to get my head around the situation with Syria.  Ten years ago, I don't think I took the debate on the Iraq war seriously enough and I'm trying to do a better job this time. 
My understanding is that Syria has been in civil war for the past couple of years.  This has involved atrocities, of course, including what our government believes was a chemical attack from the Syrian government on its own people.  A few months back President Obama said that the use of chemical weapons would be a 'red line' that 'would change the calculus' on use of US forces. 
Well, that red line has been crossed and now we need to figure out what to do. 
Let me say up front that I think that the use of chem weapons on a civilian population is outrageous and that Assad is an awful person.  If he didn't wake up tomorrow morning, I wouldn't shed any tears. 
But why are chemical weapons the red line?  If he'd simply have stayed with using ordinary bullets would he be free to simply go ahead forever?  And does that same red line exist for every country around the world?  If some dictator in Africa uses Sarin gas, will he get a visit from US forces?  What happens if Russia uses them in one of their many territorial squabbles? 
Ok, realpolitik and all of that.  We can't be everywhere and can't oppose everyone.  I get the coldness of that and it clashes with the attempt at humanitarianism at play here.  And while we're talking about clashes, it should be noted that Assad isn't as bad as the Hussein family in Iraq and yet somehow that war was opposed by some of the same people arguing for this one.
But Assad is bad.  Are we going to remove him from power then?  No.  Regime change isn't being argued for.  We have openly stated that we aren't seeking to change the regime here.  So it's back to guns and fire and explosives, but no more chem weapons.  Doesn't that give you a warm fuzzy? 
In fact, that's the big problem at the heart of this.  We're talking about launching some missiles to degrade the Syrian military, but we don't want to knock out the current government.  That is, if we completely achieve all operations as planned, Assad will still be in power and the civil war there will still rage on.  Got that?
So we have almost no upside.  Is there a downside?  Of course there is!  First of all, there will be 'collateral damage' and civilian deaths.  There will be mistakes of intelligence and blow back.  These are near certainties in war so the calculation always has to be if this is worth it.  Can anyone argue with a straight face that our objectives here are worth it? 
And this is just the simple, obvious problems.  There are other dangers at play.  Right now the situation in Syria itself is awful.  What if US involvement turns a civil war into a regional one?  Iran has already threatened to attack Israel if we go in.  Hopefully that is just bluster, but who knows? 
And what happens with the other large powers on the globe?  Russia has connections with Syria.  What if this turns into a proxy war between the US and Russian puppets.  On some scale, that's a rerun of the worst parts of the cold war.  It's been noted that World War One started with a small event and because of tangled alliances, became something huge.  That's always a danger and it isn't that hard to trace the tangles here. 
Ok, but what happens if we don't act?  The civil war continues, possibly with more chemical attacks.  More people die and refugees flee Syria.  (Which may not be at all different from what would happen if we do attack.)  Obama looks foolish because his comment about a 'red line' isn't backed up.  The US loses some credibility because of that.
The last point is a real one and it will have some negative consequences, but honestly, can you imagine going to war so that the President doesn't look bad?  How can we possibly justify that?  That might be the worst war rationale in the history of the country. 
I can't support this war.  I can't support this attack.  There is virtually no upside and the downsides are too big.  I don't really like that Obama (and the US) will lose some credibility on the world stage.  But so be it.  That can't be more important than the act of going to war.  It simply can't.