British protectorate from 1900 until 1970.
On June 4, 1970, Tonga reasserted its independence, and ended its status as a protected state.
It is now one of the few nations in the Commonwealth that has its own sovereign monarch, rather than having the British monarchy as its head of state.
If you'd like to explore Tongan history further, why not look for some books in One Search?
When you look for books in One Search, it's always a good idea to use the Refine Your Search options down the side. If you just search for "Tonga" and "History", you get all sorts of information (including a book about the history of the ukulele). But when you use the Refine options, you can narrow your results to a more on-topic group.
Did you know you can refine your search to books about particular geographic areas by selecting Regions?
Here's a search we ran for Tonga* AND history, narrowed to Books/eBooks, the discipline of History & Archaeology, and the regions of Oceana and Tonga.
Oh, and here's that book about the history of the ukelele, just because ukuleles are awesome.