That's not a supra-national authority. That's a supra-national
force. An authority is something people listen to,
without it forcing them to do so. Something people
give power over to, rather than having it taken from them.
An elected leader is an authority. A valid one-world government would presumably have authority.
A military superpower is not an authority. It's just a force. Every time it "unilaterally" makes a decision, it has to back that up with renewed force. As soon as the force stops, its control stops. That's not authority. (Think: the Byzantine empire. It had force over much of Western Europe for a few hundred years; but the people there never saw it as a valid authority. So as soon as its ability to project force ended, it lost control of its Western-European assets, and became the Eastern Byzantine Empire.)
Also, a "supra-national" body like the International Court of Justice isn't an authority or a force; it's just a hub for communication. It has no command over anyone. The only thing it does is come up with ideas, and then — if UN member countries like — they can take those ideas for their own (i.e. sign on to a given declaration to give it force within their borders.)
Such bodies are basically like the IETF, issuing RFCs. There's nothing requiring anyone to implement an RFC. You just implement it if-and-when you think it's a good idea.
Countries are top-level; nobody has true authority over them. That's what "sovereignty" means. You only have a true supra-national authority if sovereignty ceases to exist.
> You really think they "felt shamed"? They don't care about your morality in the slightest.
Adults don't generally change their ethical norms in response to exposure to external systems of ethics, no. But children often do.
This is why many countries (e.g. the US) think that putting their media out on the global stage is important: it allows them to influence the political/ethical views of children in other countries in ways that make them think more like the origin culture of that media; and so make the country more likely to invite their cooperation and influence two or three generations later, when those children are now the country's leaders.
There is a reason that e.g. America had to fight for sovereignty, but Canada did not. Or why France needed a series of internecine wars and bloody revolutions to establish democracy, but many neighbouring countries to France then peacefully, gradually established democratic societies some time later. Cultures are object-lessons for their neighbours!
> The fight continues today as well. Slavery goes on.
Well, yeah. Precisely because there isn't a supra-national authority with any authority to tell them to stop. There's just people with guns. You don't have to listen to guns if you have your own guns; especially if the guy that would point the guns at you has too many other higher-priority people to point their guns at first.