This treaty brought to an end over 20 years of increasing disagreement over conflicting territorial claims in the eastern part of Central Africa, where Portugal had long-standing claims based on prior discovery and exploration but where British citizens set up missions and embryonic trading concerns in the Shire Highlands in what is now Malawi from the 1860s.
These disagreements were increased in the 1870s and 1880s, firstly by a dispute over a British claim to part of Delagoa Bay and by the failure of bilateral negotiations between the two countries over the boundaries of Portuguese territories and secondly as a result of the Berlin Conference of 1884–85, which set-out the doctrine of effective occupation.
For these reasons, and in response to a minor armed conflict in the Shire Highlands, Lord Salisbury issued the 1890 British Ultimatum which required Portugal to evacuate the areas in dispute.
Lord Salisbury refused the Portuguese request for arbitration and, after an abortive attempt to fix the boundaries of their respective territories in 1890, the Anglo-Portuguese Treaty of 1891 was accepted by Portugal under duress.
[7] The 1879 treaty was never ratified, and in 1882 Portugal occupied the lower Shire River valley as far as the Ruo and again asked the British government to accept this territorial claim.
[8] Further bi-lateral negotiations, led to a draft treaty in February 1884, which would have included British recognition of Portuguese sovereignty over the mouth of the Congo in exchange for freedom of navigation on the Congo and Zambezi rivers but the opening of the Berlin Conference of 1884–85 ended these discussions, which could have led to British recognition of Portuguese sovereignty stretching across the continent.
[10] To establish further Portuguese claims, Serpa Pinto was appointed as its consul in Zanzibar in 1884, with a mission to explore the region between Lake Nyasa and the coast from the Zambezi to the Rovuma River and secure the allegiance if the chiefs in that area.
In 1885, the Portuguese Foreign Minister prepared what became known as the Rose Coloured Map representing a claim stretching from the Atlantic to the Indian Ocean.
This was rejected by Portugal because the Shire Highlands and their missions could at that time only be accessed through coastal areas acknowledged as Portuguese, and because the proposal would involve giving up the southern and more valuable half of the transcontinental zone of the Rose Coloured Map for little in return.
This time, it was the British government that rejected the proposal, firstly because of the opposition of those supporting the Scottish missions, and secondly, because the Chinde River entrance to the Zambezi had been discovered in April 1889.
[19] In 1888, the Portuguese government instructed its representatives in Mozambique to make treaties of protection with the Yao chiefs southeast of Lake Nyasa and in the Shire Highlands.
[20] The expedition led by Pinto was well armed, partly in response to a request from the Portuguese resident on the lower Shire for help in resolving disturbances caused by the Makololo chiefs.
The Makololo had been brought into the area by David Livingstone as part of his Zambezi expedition, and remained on the Shire north and west of the Ruo river when it ended in 1864.
Agents for Rhodes were active in Mashonaland and Manicaland and in what is now eastern Zambia, and John Buchanan asserted British rule in more of the Shire Highlands.
Talks started in Lisbon in April 1890, and in May the Portuguese delegation proposed joint administration of the disputed area between Angola and Mozambique.
Its western border with Northern Rhodesia was fixed in 1891 at the drainage divide between Lake Malawi and the Luangwa River by agreement with the British South Africa Company, which governed what is today Zambia under Royal Charter until 1924.
[33] Differing interpretations of the treaty language by the governments of the UK and Portugal revived a dispute over the boundary through the Manica Plateau, which lies between the Zambezi and Save rivers.