Well this is all opinion but we could assume there would be several Superpowers by 2077. They would be...
1) The United States
2) The European Commonwealth
3) Brazil
4) India
5) China
6) The USSR
The nuclear exchange may have been as follows.
The US and China. (We know this happens)
The European Commonwealth and The USSR. (The USSR seeking the opportunity to annex Europe or a European first strike to avoid that happening)
Brazil and China. (Brazil will almost certainly be a nuclear superpower by 2077 and probably allied with the US, Brazil would respond to the attack on the States by launching at China, China may be too commited to counter this).
India and China. (Bordering rivals)
China and the US, USSR and India.(Those guys have got their hands full!)
The USSR and Europe and China. (Despite both being communist countries, the Russians and the Chinese can't stand each other!)