Look, we can only go by what we're told. We, the people, have no ability at all to ascertain the veracity of assertions of this nature made by our government. It's not that we're naive, it's that we are in no position to go to Syria and watch every second of the hostilities so that attribution can be made independently of government agencies and the media.
If Assad did do it, then we are simply responding to what he did. If Assad didn't do it, well, it's not the first time our government has lied to us. Either way, this military action will not have much effect on the outcome of the conflict so it's not as if we're messing anything up too bad.
That's a horrible way to go about modeling reality.
What the government says is only tiny portion of the information we have on any situation.
We also have what 200 other governments say, what academics say, what thousands of different journalists say, what historical precedent says, and what your own logic and intuition says.
Your obedience to the government narrative makes you the perfect tool of those in power.
If Assad did do it, then we are simply responding to what he did. If Assad didn't do it, well, it's not the first time our government has lied to us. Either way, this military action will not have much effect on the outcome of the conflict so it's not as if we're messing anything up too bad.