What's the motive?

We have nothing to gain from conquering Syria or crushing Assad. It has nothing we want, no resources. In fact, from a purely "imperialistic" and strategic point of view, it is in our best interest (and that of our allies), that a weakened Assad remains in power. The last thing we want is another chaotic, failed state in the region, a haven for terrorists.

This is not Iraq, that had oil, command of the Straits, and a strategic geographical location bordering countries we wanted to intimidate.

The problem with your theory is that it makes no sense, especially if we accept your perspective of an Imperialist Amerika with a Neocon President. Think about it, if Cheney was boss, he'd be selling Assad all the Sarin he needed.