They were pretty much the successor to the post-World War 1 era mandates created by the League of Nations - which placed ex-German and Ottoman colonies under the administration of one of the Entente powers (which is why you might occasionally hear terms like "Mandatory Palestine"). After World War 2, when the League of Nations disappeared and the UN took its place, the remaining League mandates were placed under UN oversight and renamed to "UN Trust Territories."

Also, while it's only hinted at in the article, Nauru was a part of one of those post-WW1 League mandates so the bit about it becoming UN Trust Territory after WW2 isn't really all that special.

> I've never heard of a trust territory. What is that?

The United Nations realized after two world wars fought largely over attempts at empire and colonial possession, the most important thing to do to ensure no World War III happened was to name and shame colonial imperialists.

It was not successful at decolonizing as the victors of WWII (Russia, France, Great Britain, and the US) continue their colonizing to this day.

But what it was remarkably successful at doing was to change the conversation from "why don't we take more land from people already living there and claim it and them as our own" to "what gives a nation the right to take land from people already living there and claim the land and the people living there as their own?". (Crimea asks a question here. As does Hawaii. And the Falklands.etc etc)

The second question was simply never asked before the United Nation's resolution demanding self-government for all of the world. It was fought about, but never challenged on an intellectual level. The modern intellectual history of France is a conversation about that topic, when the post-modernism started to think about what the hell France was doing in Africa and Asia, because the UN made a point about colonialism, sovereignty, and self-determination. Not much difference between what Germany did to France during WWII, and what France had been doing to a bunch of places around the world before WWII for a couple hundred years.

(To put a slightly finer point on how the winners write the story, German, Japanese, and Italian colonial possessions became UN Trust Territories, while British, French, and US colonial possessions did not immediately get recognized as illegitimately taken colonial possessions. But the conversation that maybe that's just exactly what they were certainly came to be from that. And of course let us not forget that Nauru was a colony of a colony. And was obliterated by rapacious extractive colonialism)

Terriory.

It's been corrected now.