A huge failing of worldwide diplomacy? What do you want them to do?

There are only two routes out of this: Persuade Russia to stop, or persuade Ukraine to stop. To persuade Russia to stop, you have to persuade Putin to stop. But Putin won't, not with anything short of victory as perceived by him (or at least something he can sell to his people as victory). And Ukraine won't, not without their territory back. There is no common ground between them.

That leaves the options as either assassinate Putin, or intervene strongly enough so that one side or the other receives irrevocable military defeat. So, concretely, what exactly do you think the world should have done??