Seems like Trump called it right - step out of the way and let Russia and Turkey fight over Syrian scraps.
Turkey attacking Kurds is, more or less, expected and the world will take little interest. But Turkey attacking Syria is a different conflict, which would bring in all of Syria’s allies; Russia, Iran and a host of others who have an interest in the area. Since Turkey initiated the conflict, I’d say NATO has no role.
Like Saudi Arabia, Turkey appears to be a military powerhouse. But, like Saudi Arabia, it’s a boxer with a glass jaw. One good push and Turkey is likely to collapse into a bar-fight of revolutionary groups.
It is in the interests of the US, and all civilized people, for Assad and Russia to regain total control over the entirety of the Syrian territory.
If Turkey invades, I would support the Russian Air Force taking out all their armor and troop concentrations.