I don't disagree with you but I do wonder whether for sanctions to be successful relies a lot on the way the population is feeling about the regime at the start. If they population decides to dig in, then the sanctions start to have a negative effect (in terms of getting a desired outcome).

If on the other hand the population is already angry at the regime, maybe it has the desired effect.

Idle musing without supporting evidence.

Only the US sanctioned Cuba and while it was pretty harmful, they were still able to trade with most other countries. North Korea has China right next door as a big trade partner, and can trade with many non-US countries as well. As for Iran, from what I can tell UN sanctions really only started to ramp up in the 2010s and those did have an effect in getting Iran to suspend its nuclear program.

Edit: But to be clear, I think the US sanctions on Cuba are gratuitous and cruel. Cuba is not some rogue state and there are many worse dictatorships that aren't sanctioned. However, I think it reinforces the point that "sanctions can be devastatingly effective applied by a significant majority of the world" because in the Cuba case the rest of the world didn't agree.

> Only the US sanctioned Cuba and while it was pretty harmful, they were still able to trade with most other countries.

There were considerable limits (particularly on foreign investment in Cuba) because parts of the US sanctions regime include retaliatory sanctions on entities making use of certain property in Cuba (some of these were suspended for a while, but restored under Trump in 2017.)