In the case of Syria, we are not welcome in their country or over their airspace. We levied a significant act of war against Syria and its currently legitimate government (no matter how our government feels about Assad).
Let's turn the tables to try and see the enormity of what he did. Granted the USA has done this sort of thing up to hundreds of times over the last couple of decades but that doesn't legitimize it. Imagine if Russia, who is allied with Syria just as we are allied with Great Britain, the EU, Japan, South Korea, etc. brought the war to us. What if they attacked the two destroyers involved in the attack and succeeded in sinking them, and then dropped tactical nukes on their home base. By my way of thinking, that would be a responsible response to an act of war against a close ally. How would we feel about that? Would we understand that we were the aggressor that began the whole thing? If something like that were to happen and the tit-for-tat progressed no further, would taking out a hand full of airplanes, some fuel, and some buildings with 49 cruise missiles have been worth the cost? We've been so insulated within our borders that we fail along with our political leaders to understand that acts of war can come with great consequences against us, and perhaps justifiably. And that is why I believe we should refrain from such acts except for when we are directly threatened, and then we shouldn't do it in tiny steps, we should do it in enormity and to full completion. There should be nothing left of that threat when the dust settles, and the whole time we must be vigilant and prepared to fight for our survival against any nation-state(s) who might retaliate against us for our actions.