Tonga became a British protected state under a Treaty of Friendship on 18 May 1900, when European settlers and rival Tongan chiefs tried to oust the second King.

The Treaty of Friendship and protected state status ended in 1970 under arrangements established prior to her death by the third monarch, Queen Sālote.

An unspoken agreement of the treaty that was common in British protected states was a new monopoly on Tonga's thriving vanilla industry, and their small deposits of minerals.

Tonga's complex social structure is essentially broken into three tiers: the King, the nobles, and the commoners.

The end of the Kingdom of Tonga protected state status was arranged by Salote Tupou III prior to her death in 1965.