[quote name='Hex']Okay, so by that logic, say there was a mexican paramilitary group acting inside the US' borders conducting terrorisms against mexico. Mexico launches a strike that kills a handful of innocent people in Texas.
Is bombs away still an okay tactic when it's your own

ing people being killed?[/QUOTE]
Lets look at history. When Pancho Villa's forces were attacking the US from Mexico, the US sent 10,000 troops into Mexico to stop him.
Ignoring the absurdity of your hypothetical situation, if one nation is harboring forces which are attacking your nation, whether through official support, tacit support, or negligence, the nation under attack has every right to defend itself and go after those forces.
Lines on a map are just lines on a map, unless they've got the ability and the will to back it up. And if they could do that, eliminating the 3rd party is well within their means, in which case they are supporting the forces attacking you. Which isn't any different than that nation sending its own troops in to attack you.
Syria is unhappy that their sovereignty has been violated. But Syria has been violating Iraq's sovereignty on a daily basis by allowing these foreign fighters to enter Syria, arm and organize in Syria, and enter Iraq.
If Syria can't or won't stop these people, and they can't stop you, why would you care what Syria thinks?