The rivers of Guinea and the islands of Cape Verde were among the first areas in Africa explored by the Portuguese, notably Nuno Tristão, 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 local African rulers in Guinea, who prospered greatly from the African slave trade, had no interest in allowing the Europeans any further inland than the fortified coastal settlements where the trading takes place. 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 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 Bijagós 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 Amílcar Cabral and Rafael Barbosa. The PAIGC moved its headquarters to Conakry, Guinea, in 1960 and started an armed rebellion against the Portuguese in 1961 (for a detailed account of this struggle, see the PAIGC page). 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, General António de Spínola, returned to Portugal and led the movement which brought democracy to Portugal and independence for its colonies.
Amílcar 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 24 September 1973 and was recognized by a 93-7 UN General Assembly vote in November, unprecedented as it denounced illegal Portuguese aggression and occupation and was prior to complete control and Portuguese recognition. Following Portugal's April 1974 Carnation Revolution, it granted independence to Guinea-Bissau on 10 September 1974. Luís Cabral, Amílcar Cabral's half-brother, became President of Guinea-Bissau.
Following Portugal's April 1974 Carnation Revolution, it granted independence to Guinea-Bissau on 10 September 1974. Luís Cabral, Amílcar Cabral's half-brother, became President of Guinea-Bissau. Following independence local soldiers that fought along with the Portuguese Army against the PAIGC guerrillas were slaughtered by the thousands. A small number escaped to Portugal or to other African nations. The most famous massacre occurred in Bissorã. In 1980 PAIGC admitted in its newspaper "Nó Pintcha" (dated 29/11/1980) that many were executed and buried in unmarked collective graves in the woods of Cumerá, Portogole and Mansabá. In late 1980, the government was overthrown in a relatively bloodless coup led by Prime Minister and former armed forces commander João Bernardo Vieira.
From November 1980 to May 1984, power was held by a provisional government responsible to a Revolutionary Council headed by President João 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.
From 1980 to 1991, opposition parties were illegal. Aristide Menezes led the Democratic Front, which in 1991 became the first legal opposition party and paved the way for democratic elections.
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, 20 years after independence from Portugal, the country's first multiparty legislative and presidential elections were held. An army uprising that triggered the Guinea-Bissau Civil War in 1998, created hundreds of thousands of displaced persons. The president was ousted by a military junta in 7 May 1999. An interim government turned over power in February 2000 when opposition leader Kumba Ialá took office following two rounds of transparent presidential elections. Guinea-Bissau's transition back to democracy has been complicated by a crippled economy devastated by civil war and the military's predilection for governmental meddling.
In September 2003 a bloodless coup took place in which the military, headed by General Veríssimo Correia Seabra, arrested Ialá, because "he was unable to solve the problems". After being delayed several times, legislative elections were held in April 2004.
A mutiny of military factions in October 2004 resulted in the death of General Seabra and others, and caused widespread unrest. The Prime Minister Carlos Gomes Júnior has stated that the mutineers were ex-UN soldiers recently returned from Liberia who were angry about delays in being paid. Talks between these soldiers and the authorities have so far failed to come to an agreement.
In June 2005, presidential elections were held for the first time since the coup that deposed Ialá. Ialá returned as the candidate for the PRS, claiming to be the legitimate president of the country, but the election was won by former president João Bernardo Vieira, deposed in the 1998 coup. Vieira was a candidate for one sect of the PAIGC. Vieira defeated Malam Bacai Sanha in a runoff-election, but Sanha refused initially to concede, claiming that the elections had been fraudulent in two constituencies, including the capital Bissau.
Despite reports that there had been an influx of arms in the weeks leading up to the election and reports of some 'disturbances during campaigning' - including attacks on the presidential palace and the Interior Ministry by as-yet-unidentified gunmen - European monitors labelled the election as "calm and organized". 
On 2 March 2009, Vieira was assassinated by soldiers. This may be part of another coup attempt. While on 5 June, several major politicians (Baciro Dabo, Faustino Imbali and Helder Proenca) were shot to death, officially to counter a planed coup d'État against the temporary military leadership.