You do have some impact, as a voter. E.g. you might not be personally afraid of ISIS but the threat they pose to the region and Europe might be enough for you to think that the US should reverse its policy of trying to overthrow Assad.
But I think that this is something an ordinary citizen can do something about. The policy up until now was not created by arms manufacturers, or by neo-cons alone, but primarily by moderate liberals. Noam Chomsky or Norman Finkelstein would say that these moderate liberals had been tricked into acting as pawns for the neo-cons. That doesn't have the ring of truth to me. I think they act according to their own ideology, albeit a hypocritical one that is liberal at home and right wing on the Middle East. This ideology can be argued and lobbied against by people with different opinions.