Though the country had claimed the area four years earlier, Portuguese explorer Nuno Tristão sailed around the coast of West Africa, reaching the Guinea area in about 1450, searching for the source of gold, other valuable commodities, and slaves, that had slowly been trickling up into Europe via land routes for the preceding half century.
It will never be known exactly how many human lives were bought and sold in the slave markets along the Guinea coast (mostly by the Portuguese; 37% of all slaves imported from Africa were bound for the Brazilian colonies), but it is today approximated at 10 million. Cacheu, in Guinea-Bissau, was one of the largest slave markets in Africa for a time.
After the abolition of slavery in the late 1800s, the slave trade went into serious decline, though a small illegal slaving operation continued. Bissau, founded in 1765, became the Portuguese Guinea colony's capital.
Though the coast had been under firm Portuguese control for the past four centuries, it was not until the Scramble for Africa that any interest was taken in the inland part of the colony.
A large tract of land that was formerly Portuguese was lost to French West Africa, including the prosperous Casamance River area, which had been a large commercial centre for the colony. Britain tried to take control of Bolama, which lead to an international dispute that came close to war between Britain and Portugal until U.S. president Ulysses S. Grant intervened and prevented a conflict by ruling that Bolama belonged to Portugal.
Portuguese Guinea was administered as part of the Cape Verde Islands colony until 1879, when it was separated from the islands to become its own colony.
At the turn of the 20th century, Portugal began a campaign against the animist tribes of the interior, with the help of the coastal Islamic population. This began a long struggle for control of both the interior and remote archipelagos: it would not be until 1936 that areas like the Bijagos Islands would be under complete government control.
In 1951, when the Portuguese government overhauled the entire colonial system, all Portugal's colonies, including Portuguese Guinea, were renamed "overseas provinces".
The fight for independence began in 1956, when Amílcar Cabral founded the Partido Africano da Independência da Guiné e Cabo Verde (African Party for the Independence of Guinea and Cape Verde), the PAIGC. The PAIGC was a relatively peaceful movement until 1961, when it launched a full scale guerrilla war against the Portuguese, declaring the overseas province independent and renaming it Guinea-Bissau.
The war began to turn against the Portuguese, and following the coup d'état in Portugal in 1974, the new government began to negotiate with the PAIGC. As his brother Amílcar had been assassinated in 1973, Luís Cabral became the first president of independent Guinea-Bissau after independence was granted on September 10, 1974.