The rivers of Guinea and the islands of Cape Verde were among the first areas in Africa explored by the Portuguese in the 15th century. Portugal claimed Portuguese Guinea in 1446, but few trading posts were established before 1600. In 1630, a "captaincy-general" of Portuguese Guinea was established to administer the territory. With the cooperation of some local tribes, the Portuguese entered the slave trade and exported large numbers of Africans to the Western Hemisphere via the Cape Verde Islands. Cacheu became one of the major slave centers, and a small fort still stands in the town. The slave trade declined in the 19th century, and Bissau, originally founded as a military and slave-trading center in 1765, grew to become the major commercial center.

Portuguese conquest and consolidation of the interior did not begin until the latter half of the 19th century. Portugal lost part of Guinea to French West Africa, including the center of earlier Portuguese commercial interest, the Casamance River region. A dispute with Great Britain over the island of Bolama was settled in Portugal's favor with the involvement of U.S. President Ulysses S. Grant.

Before World War I, Portuguese forces, with some assistance from the Muslim population, subdued animist tribes and eventually established the territory's borders. The interior of Portuguese Guinea was brought under control after more than 30 years of fighting; final subjugation of the Bijagos Islands did not occur until 1936. The administrative capital was moved from Bolama to Bissau in 1941, and in 1952, by constitutional amendment, the colony of Portuguese Guinea became an overseas province of Portugal.

In 1956, the African Party for the Independence of Guinea and Cape Verde (PAIGC) was organized clandestinely by Amilcar Cabral and Raphael Barbosa. The PAIGC moved its headquarters to Conakry, Guinea, in 1960 and started an armed rebellion against the Portuguese in 1961. Despite the presence of Portuguese troops, which grew to more than 35,000, the PAIGC steadily expanded its influence until, by 1968, it controlled most of the country.

It established civilian rule in the territory under its control and held elections for a National Assembly. Portuguese forces and civilians increasingly were confined to their garrisons and larger towns. The Portuguese Governor and Commander in Chief from 1968 to 1973, Gen. Antonio de Spinola, returned to Portugal and led the movement which brought democracy to Portugal and independence for its colonies.

Amilcar Cabral was assassinated in Conakry in 1973, and party leadership fell to Aristides Pereira, who later became the first president of the Republic of Cape Verde. The PAIGC National Assembly met at Boe in the southeastern region and declared the independence of Guinea-Bissau on September 24, 1973. Following Portugal's April 1974 revolution, it granted independence to Guinea-Bissau on September 10, 1974. The United States recognized the new nation that day. Luis Cabral, Amilcar Cabral's half-brother, became President of Guinea-Bissau. In late 1980, the government was overthrown in a relatively bloodless coup led by Prime Minister and former armed forces commander Joao Bernardo Vieira.

From November 1980 to May 1984, power was held by a provisional government responsible to a Revolutionary Council headed by President Joao Bernardo Vieira. In 1984, the council was dissolved, and the National Popular Assembly (ANP) was reconstituted. The single-party assembly approved a new constitution, elected President Vieira to a new 5-year term, and elected a Council of State, which was the executive agent of the ANP. Under this system, the president presides over the Council of State and serves as head of state and government. The president also was head of the PAIGC and commander in chief of the armed forces.

There were alleged coup plots against the Vieira government in 1983, 1985, and 1993. In 1986, first Vice President Paulo Correia and five others were executed for treason following a lengthy trial. In 1994, the country's first multi-party legislative and presidential elections were held. An army uprising against the Vieira government in June 1998 triggered a bloody civil war that created hundreds of thousands of displaced persons. The president was ousted by a military junta in May 1999. An interim government turned over power in February 2000 when opposition leader Kumba Yala, founder of the Social Renovation Party (PRS), took office following two rounds of transparent presidential elections.

Despite the elections, democracy did not take root in the succeeding three years. President Yala neither vetoed nor promulgated the new constitution that was approved by the National Assembly in April 2001. The resulting ambiguity undermined the rule of law. Impulsive presidential interventions in ministerial operations hampered effective governance. On November 14, 2002, the President dismissed the government of Prime Minister Alamara Nhasse, dissolved the National Assembly, and called for legislative elections. Two days later, he appointed Prime Minister Mario Pires to lead a caretaker government controlled by presidential decree until the September 14 military intervention. The Constitution provides for an independent judiciary and for the Supreme Court to choose its own leadership; however, it was subject to political influence and corruption, and was undermined when President Yala replaced the President of the Supreme Court on two occasions in 2002.

Elections for the National Assembly were scheduled for April, but later postponed until June and then October. On September 12, the President of the National Electoral Commission announced that it would be impossible to hold the elections on October 12 as scheduled. The army, led by Chief of Defense General Verrisimo Correia Seabra, intervened on September 14. President Yala announced his "voluntary" resignation and was placed under house arrest. The government was dissolved and 25-member Committee for Restoration of Democracy and Constitutional Order was established. On September 28, businessman Henrique Rosa, was sworn-in as President. He had the support of most political parties and of civil society. Artur Sanha, PRS President, was sworn-in as Prime Minister.