Verified answer

More than likely expel some diplomats, perhaps close down an embassy or two, and impose sanctions. 

Why would he tip the Russians on his attack?

He won't do a goddamn thing 

I'm sure it will involve sanctions. 

You know sanctions, those things Flynn got indicted for lying about using as a bargaining chip?

Biden will negotiate all the deals Trump didn't because he can't negotiate his way out of a paper bag.

Sanctions is the easiest way to go.

Last time I checked, the United States was still a major economic power, despite a crazy decade accelerating.

That would mean getting traditional American allies (Canada, western Europe, Japan, Australia) on board. Biden is likely to easily achieve consensus.