Tonga is governed under the constitution of 1875 as revised. The monarch is the head of state, and the government is headed by the prime minister, who is elected by the Legislative Assembly. The unicameral Legislative Assembly has 26 seats, 9 for nobles and 17 for representatives elected by popular vote; all serve four-year terms. Tonga is a member of the Commonwealth of Nations. Administratively, the country is divided into the three island groups.

Archaeological evidence indicates that the islands of Tonga were settled c.830 b.c., but the Polynesians are believed to have arrived some 400 years after that. The current ruling dynasty traces its rise to power to the 10th cent. Dutch navigators sighted the northern islands in 1616 and the rest of the group in 1643. Capt. James Cook visited the islands in 1773 and 1777 and named them the Friendly Islands. English missionaries arrived in 1797 and helped to strengthen British political influence. Internal wars in the early 19th cent. ended with the accession of King George Tupou I (1845-93), who unified the nation and gave it a constitution (1862), a legal code, and an administrative system. His successor, King George Tupou II (1893-1918) concluded a treaty making Tonga a British protectorate in 1900. Tonga remained self-governing, with the British responsible for foreign and defense affairs. Queen Salote Tupou III ruled from 1918 to 1965, when she was succeed by her son, King Taufa'ahau Tupou IV. A new treaty in 1968 reduced British controls, and complete independence was attained on June 4, 1970.

Since the late 1980s, Tongans have agitated for democratic reforms, but the king has generally opposed any change that would dilute the monarchy's power. In 2001 it was revealed that as much as $37 million in government funds had disappeared as a result of investment in a Nevada asset management company, and corruption within the royal family and government remains a problem. Amendments in 2003 to the constitution permit the restriction of freedom of speech, a move that was used to silence publications critical of the government, but parts of the amendments (and restrictive media laws passed in 2003) were subsequently declared void.

In 2005 two commoners were selected to join the cabinet for the first time, and in 2006 one (Fred Sevele) was appointed prime minister, also a first. In July-Sept., 2005, the nation experienced a civil service strike that turned into a call for democratic reform, but the strike was settled without any addressing of the broader political issues. King Taufa'ahau Tupou IV died in 2006, and George Tupou V succeeded him.

Frustration over the failure of the legislature to enact reforms led to rioting in the capital in Nov., 2006; many government offices and businesses were destroyed. Following the rioting, the government imposed a state of emergency that was not rescinded until Feb., 2011, and announced that there would be new legislative elections in 2008 and that a majority of the members of the legislature would be popularly elected. Subsequently, the government arrested a number of prodemocracy legislators on charges relating to the riots and moved to set back legislative reform to as late as 2010.

In the 2008 legislative elections, prodemocracy candidates, including incumbent legislators facing sedition charges dating from the 2006 riots, won two thirds of the popularly elected seats. In July, 2008, prior to the king's formal coronation, he announced that he would yield much of his power as part of a move toward democracy, but progress toward that goal was slow. The five legislators accused of seditious conspiracy had all their indictments dismissed in Sept., 2009, except for a seditious speech charge against one representative. A tsunami the same month devastated the northern island of Niuatoputapu. In Nov., 2009, the constitutional commission issued its recommendations, which called for reducing the king's power, making the head of government answerable to the Legislative Assembly, and increasing the people's legislative representatives; in Apr., 2010, legislation increased the number of the people's representatives in future elections. In July, however, the judicial independence was undermined when the king was given control over the appointment of judges. In November, a prodemocracy party won a majority of the popularly elected seats, but an alliance of the noble representatives and independent representatives chose the new prime minister, Lord Tuivakano. In Mar., 2012, the king died; his brother succeeded him as Tupou VI.