Advances in medical technology + drones = US will never suffer mass casualties ever again. If we fought the second gulf war with Vietnam-era technology, I bet US casualties would've been an order of magnitude higher.
You can't fuck with countries forever, eventually they will find a way to hit back on you.
Colonial powers had to learn it the hard way, and just like the US, they tried to put a price tag that was scary high on any casualties they had, and it can work for a while, but if you fuck with people long and hard enough, eventually they'll figure out they have nothing to lose.
And that road is fucking scary, because if you think 9/11 is the worst that can happen, I think you are a crazy optimist.
Either that or they'll get nukes, because that's the only other thing the US have shown will stop them from bombing you.