In a scenario where the USA puts up a national firewall preventing miners/clients in the USA connecting with miners/clients elsewhere,
The US government would have to go through a considerable legal fight to put such thing in place.
But, assuming they've done it (or assuming we're talking about a shameless dictatorship), and assuming they want to forbid bitcoin, they don't need to stop there (forbid international connections).
It's relatively easy to track down most bitcoin nodes, as is the case with all p2p networks I'm aware of. So, a totalitarian government trying to block a particular p2p network would not only block foreign access, but would also cut Internet access of every internal IP it sees running a node in this p2p network - that's what France is doing right now, cutting Internet access of those who use p2p file sharing to upload copyrighted content. In the case of France it seems the effect is more modest since they are required to prove that you actually was uploading a particular forbidden content, and they don't cut your access immediately, it seems you're given some warnings before. A more unscrupulous government could just shut you down for running the p2p node. Eventually even put a few people in jail just to make an example.
Using p2p darknets as Tor or I2P to avoid being seen may not be enough, as the same attack could be done against such networks - I read somewhere that it's already very difficult to access Tor from within China, as all relays and most bridges are blocked.
That would make running such p2p node so difficult and dangerous inside this jurisdiction, that only a small percentage of the population would have the technical skills to do it - not to mention the guts to take the risk.
It's sad, but while governments can control ISPs, they can do considerable damage, even to p2p networks as bitcoin.