What's frightening is what will Syria's response be?
Syria's been upgrading its military through Russia lately, but most of its technology is still pretty old. 1960s-1970s Soviet Era tanks, artillery, etc. Syria could pose a serious threat to only 150,000 US troops on the ground. If Syria invaded Iraq in response to this, Israel would get involved. If Israel got involved, Iran would get involved. You would have the ultimate Sunni vs. Shi'ia war. I'm sure Turkey would find an excuse to unleash their military on the Kurds like they've been wanting to do. Jordan, Saudi Arabia, Yemen, Qu'tar, The UAE, would probably remain neutral since Israel would be involved. It could destabalize Afghanistan and Pakistan, which could lead to a conflict with India. If any of the Eastern European countries got involved, Russia would join in. If that happened, Western Europe would get involved. We would essentially have World War 3.
Syria has been holding peace talks with Israel for a while. It could call off the talks and expell the US ambassador to Syria. Syria also controls Lebanon and Iran controls Syria.
My gut reaction tells me this was beyond stupid. If it was Bin Laden that we killed, ok, whatever... but honestly, the US has been making almost daily cross border raids with Pakistan. Pakistan has been ignoring it. If Syria ignores this, they will look weak. Syria just got owned last year by israel when Israel took out a nuclear reactor. And all Syria did was complain. If Syria just complains again, this will encourage further attacks by the US. Assad will look weak if he doesn't do anything and there will be a bounty on his head from the radicals and Syria could turn into another Iran.
What we've done is handed power to Syria waiting for their response. What we've also done is violated the sovreignty of another nation. You can make the excuses for Afghanistan, we had a UN resolution supporting that. You have a problem with Iraq, but right now we're there with the new government's permission. We've had drones go into Pakistan, but this is the first time since Iraq that the US military has had actual soldiers on the ground of another foreign country. You can't just take matters into your own hands here and make cross border raids into Syria.