2) Obama better go in with eyes wide open: the rebels we're arming are NOT white hats. We're facing a situation where we may end up forging another wingnut group like the Taliban that took over Afghanistan after we supplied them with weapons to repulse the Soviets. If we're going to go in to buy this war, we better be prepared to own it.
3) Mission Creep. When you go in without a clear set of goals, when you go in without a planned exit or alternative solutions when your primary goal gets f-cked in the fog of war, you get mission creep where you keep re-justifying just what the hell you're doing when you're out of options.
This was what happened with Vietnam: our objectives kept changing, our "needs" (actually our political leaders' fear of losing) kept growing, until we got a war we didn't really want or need. This was Lebanon: we went in thinking just our military presence would calm thing down, it didn't, Marines died and Reagan got us the hell out. This was Somalia: we went in for humanitarian reasons to end a famine, it changed into trying to stop the in-fighting between factions, got into a firefight we couldn't control, Clinton ended up bugging out (Somalia's still a war-torn mess). Afghanistan was a response to 9/11 and in a way had to happen, but we went in without a clear objective or defining endgame, and we then piled a second war - the Iraq Invasion/Occupation - on top of it. And speaking of the Second Persian Gulf War, we went in with an objective - get rid of Saddam - but didn't have a smart plan in place for getting out (the plan for getting out was putting a puppet Chalabi on the throne: when the Iraqis refused him, we got stuck without a Plan B).
When we had clear goals and authority to do so - the first Persian Gulf War is as close to an example we can get in the modern era - we had some success: we went in, we got out.
If Obama's track record is any indication, we can hope (barely) that Mission Creep won't happen: his handling of Libya - minimal ground support, air support to enforce a no-fly zone - showed keen focus on objectives and sticking to them. And aside from the Benghazi attack - done by insurgents threatening the more open Libyan nation already - the gameplan seemed to work.
Right now, Obama is just arming rebels. What happens when the means for "victory" requires U.S. air support to enforce a no-fly zone over a more dangerous airspace than Libya's? What happens if Assad gets outside support (unlikely, but possible)? We face the real danger of Mission Creep...
Of great concern is that Obama is doing this without taking it to Congress: the whole argument behind the War Powers Act was to stop military adventures without oversight or responsibility. It'd be nice to think Obama could take this to Congress... except given the GOP's gameplan of obstructing Obama at every turn, I'd worry Congressional Republicans would use it as either 1) a way to attack Obama or 2) a bargaining chip to force Obama to accept a Far Right agenda (tax cuts for the rich) on the economy (or even worse a repeal of Obamacare, just so they could stick the dagger in).
For myself, I trust Obama on this: he didn't overplay his hand on Libya the way Bush the Lesser and Cheney did over Iraq. I'm just worried about the ongoing precedents of executive adventurism...