This would be devastating, both in terms of casualties and long-term economics.
Would Russia launch World War III over Syria?  Quite possibly, given its strategic and financial interests there.
So that leaves the next question: What U.S. targets and cities would Russia most likely attack?

nuclear-collapse-endHours Ago Donald Trump Ordered the Launch of What Could Rapidly Devolve into a Nuclear WW3.
Is THIS How It Ends?