I've often wondered if that is even possible (whether it is good policy or not is another question entirely). Could we disconnect Russia from the Internet effectively? Let's say that Europe could be pressured to cooperate, what then? Well, here a couple of years ago I finally got the answer I wanted: we can't. China would never abide any such sanction, and there must be a few overland backbones connecting the two (even if I'm wrong here, wouldn't take decades for those to be built).

Likely, the country that wanted to do this finds themselves isolated on their own network, not their target isolated from the internet. Even if that country as is large and powerful as the United States. Perhaps the answer might have been different, 20 years ago or even 15, but everything has changed and there's no going back.

Exactly, this would just result in a global game of whack-a-mole. It is possible in autocracies that are mostly excluded from global trade, like North Korea, but China for example can’t afford to cut itself off without collapsing its economy. (It has the Great Firewall, but that does not block entire countries, and is often quite leaky.)

It would be easy. Require the backbone CEO's to certify that their networks don't connect to networks that connect to China, Russia, Nigeria, etc. The burden would then shift to them. If they couldn't get a guarantee from a peer or customer, they would have to disconnect them.