By the 12th century Tongans, and the Tongan paramount chief, the Tuʻi Tonga, had a reputation across the central Pacific—from Niue, Samoa, Rotuma, Wallis & Futuna, New Caledonia to Tikopia—leading some historians to speak of a Tuʻi Tonga Empire. In the 15th century and again in the 17th, civil war erupted. Into this situation the first European explorers arrived, beginning in 1616 with the Dutch explorers Willem Schouten and Jacob Le Maire (who called on the northern island of Niuatoputapu), and in 1643 with Abel Tasman (who visited Tongatapu and Haʻapai). Later noteworthy European visitors included James Cook (British Navy) in 1773, 1774, and 1777, Alessandro Malaspina (Spanish Navy) in 1793, the first London missionaries in 1797, and the Wesleyan Methodist Rev. Walter Lawry in 1822.
In 1845 the ambitious young warrior, strategist, and orator Tāufaʻāhau united Tonga into a kingdom. He held the chiefly title of Tuʻi Kanokupolu, but had been baptised with the name Jiaoji ("George") in 1831. In 1875 with the help of missionary Shirley Waldemar Baker, he declared Tonga a constitutional monarchy, formally adopted the western royal style, emancipated the "serfs", enshrined a code of law, land tenure, and freedom of the press, and limited the power of the chiefs.
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. Within the British Empire, which posted no higher permanent representative on Tonga than a British Consul (1901–1970), Tonga formed part of the British Western Pacific Territories (under a colonial High Commissioner, residing on Fiji) from 1901 until 1952. Although under the protection of Britain, Tonga remained the only Pacific nation never to have given up its monarchical government—as did Tahiti and Hawaiʻi. The Tongan monarchy follows an uninterrupted succession of hereditary rulers from one family. In 1918 the influenza epidemic that spread through the world caused the deaths of 1,800 people in Tonga, approximately 8% of the population.
The Treaty of Friendship and Tonga's protectorate status ended in 1970 under arrangements established by Queen Salote Tupou III prior to her death in 1965. Tonga joined the Commonwealth of Nations in 1970 (atypically as an autochthonous monarchy, that is one with its own local monarch rather than that of the United Kingdom—compare Malaysia, Lesotho, and Swaziland), and became a member of the United Nations in September 1999. While exposed to colonial pressures, Tonga has never lost indigenous governance, a fact that makes Tonga unique in the Pacific and gives Tongans much pride, as well as confidence in their monarchical system. As part of cost cutting measures across the British Foreign Service, the British Government closed the British High Commission in Nukuʻalofa in March 2006, transferring representation of British interests in Tonga to the UK High Commissioner in Fiji. The last resident British High Commissioner was Paul Nessling.