"Heart of Darkness" begins and ends in London; on the Nellie on the Thames. The most part, however, takes place in the Congo (now known as the Republic of the Congo). The Kongo, as it was originally known, was inhabited first by pygmy tribes and migratory 'Bantus' and was 'discovered' by the Portuguese in the 14th Century. The Portuguese brought with them Catholocism; European missionaries. The Congo was ruled by King Alfonso I from 1506 - 1540 and Shamba Bolongongo from 1600 - 1620. The slave trade was rife in the Congo, from about 1500 until 1830. King Leopold of Belgium ruled, between 1878 and 1908, and would have been King at the time "Heart of Darkness" was set. Conrad himself actually arrived in the Congo on 12 June 1890, and it would be safe to say that he would have used his experience in the Congo when writing "Heart of Darkness".
At its time of writing for Blackwood's Magazine (December 1898), Britain was in its last years of Victorian rule. Queen Victoria was actually the niece of King Leopold of Belgium. Britain was the most powerful and influential nation on Earth; its Empire spread throughout Europe, Asia and Africa. Joseph Conrad, born in the Ukraine in 1857, as Jozef Teodor Konrad Korzeniowski, as the author, was an outsider looking out. Neither British nor African, he seemed to be the perfect candidate for writing about two countries he had knowledge of - England and the Congo.
African exploration was quite popular; in Conrad's day, Livingstone died in 1873, in Ilala, Africa, and Stanley returned from his final African expedition in 1890. As exploration was popular, so was the adventure story - tales of African exploration were available in abundance. Imperialism was also a popular theme at this point in the late nineteenth Century. Conrad's novella, whilst to contemporary critics (Achebe, for example) may appear racist; at the time was accepted as another piece of work from a very