In the middle of June 1850 when the steamer Sharpshooter, despatched from England to reinforce the British naval squadron on the Brazilian coast, arrived off Rio de Janeiro and immediately made its presence felt in the vicinity of the capital by capturing two Brazilian vessels–the Malteza which was destroyed on the spot and the Conceição which was sent to StHelena—the Brazilian slave trade was already being carried on less extensively than at any time for almost a decade. The frequently repeated declaration by the Brazilian government of their intention to introduce fresh measures to combat the trade, the activities of British warships along the Brazilian coast during the previous twelve months (in addition to the continued vigilance of the British West African and Cape squadrons) and, perhaps most important, the glut in the Brazilian slave market after several years of exceptionally heavy imports, had combined to reduce the Brazilian slave trade to a mere shadow of its former self. Only 8,000 slaves had been landed along the coast between Santos and Campos during the period January–June 1850, less than a third of the number imported in any comparable period during recent years. Bahia alone, where significantly not a single British cruiser had put in an appearance for almost a year, had imported its usual quota of slaves—a little over 4,000.

During the 1830s, as indeed throughout the long and costly campaign which lasted for more than half a century from 1807 to the mid-sixties, Britain's efforts for the suppression of the transatlantic slave trade at sea were concentrated on the coast of west Africa. Since 1807, when the sloops Pheasant and Derwent were first sent out to enforce Britain's own anti-slave trade legislation, a number of ships of the Royal Navy had been stationed on the west African coast where their duties had also come to include the suppression of the illegal foreign slave trade. After 1819, following the signing of the first right of search treaties with Portugal, Spain and the Netherlands, the coast from Cape Verde in the north to Benguela in the south (3,000 miles) had constituted a separate naval station. But from 1832 to 1839 the West African squadron came under the orders of the commanders-in-chief of the Cape of Good Hope station, successively Rear Admiral Frederick Warren (1831–4), Rear Admiral Sir Patrick Campbell (1834–7) and Rear Admiral George Elliot (1837–40). This combined West African and Cape station covered an enormous area from 26° W.to 75°E., and as far north as 23° 30′N. in the Atlantic and 10°S. in the Indian Ocean. Yet in 1836, for example, it accounted for only 14 of the 100 or so ships and a little over 1,000 of the 17,000 men on all foreign stations.

At the beginning of the nineteenth century when Britain launched her crusade against the transatlantic slave trade, there was no nation more deeply involved in the exportation, transportation and importation of African slaves than Portugal. Although the African territory effectively occupied or controlled by the Portuguese (who had first discovered and explored the coastline of Africa three and a half centuries before) scarcely extended beyond a few scattered towns, settlements and fortified trading posts along the coast and along the banks of the great rivers, thousands of slaves were annually exported to the New World from those parts of the African continent claimed by the Portuguese Crown as part of its vast overseas dominions. Bissão and Cacheu in what is now Portuguese Guinea, which were governed from the Cape Verde Islands, had already been virtually ‘slaved out’ and the Portuguese had long since lost to the English, the French, the Dutch and the Danes their monopoly of trade on the Costa da Mina (the coastline north of, and parallel with, the equator, stretching from Cape Palmas to the Cameroons and embracing the Ivory, Gold and Slave Coasts, sometimes collectively known as the Guinea Coast). But they retained the islands of São Tomé and Prícipe in the Gulf of Guinea, through which a great number of slaves were exported, and the fortified post of São João Baptista de Ajudá at Whydah in Dahomey, perhaps the most important slave port north of the equator.

In 1845 Lord Aberdeen had felt compelled to seek authorisation for the kind of high-handed anti-slave trade measures more readily associated with the name of Lord Palmerston. He had, however, shown a much greater concern than his predecessor for at least the façade of legality: in instructing British warships to seize all Brazilian ships engaged in the slave trade and British vice-admiralty courts to adjudicate upon them the British government, it was claimed, were merely exercising their treaty rights. And as Peel was at pains to explain to the House of Commons, it was ‘the mildest measure to which they [the British government] could resort’ they were deliberately not seeking authority from Parliament for British courts to punish Brazilian subjects engaged in the slave trade as under article I of the treaty of 1826 they had every right to do. Moreover, the decision to introduce the Brazil (Slave Trade) bill had been taken ‘with pain and only under the last necessity’. At the time of its passage Aberdeen offered to repeal it, either when the slave trade had finally ceased or when the Brazilian government signed an anti-slave trade treaty acceptable to Britain.

When in March 1845 the Brazilian government had professed a willingness to enter negotiations for a new anti-slave trade treaty they had done so in the belief that by terminating the right of search treaty of 1817 they had seized the diplomatic initiative and put themselves in a strong position to dictate its terms: the British government would now have to accept Brazil's treaty proposals, they reasoned, or else abandon entirely their efforts to suppress the Brazilian slave trade.

As Captain Joseph Denman, one of the most experienced British naval officers to serve in the anti-slave trade blockade of the west African coast, was later to recall, the year 1839 opened ‘an era in the history of the slave trade when for the first time suppression [by the British navy] became possible’. Ships of the Royal Navy were at last empowered to search and capture Portuguese and Brazilian (as well as Spanish) vessels—together with those without nationality—carrying slaves or simply equipped for the trade in southern as well as northern latitudes. Moreover, the activities of the navy—at least on the African side of the Atlantic where Britain's efforts for the suppression of the foreign transatlantic slave trade were still concentrated—were no longer confined to the high seas. Lord Palmerston had already advised the Admiralty that, in the opinion of Sir John Dodson, the Advocate General, if British warships were to enter African waters and rivers it was unlikely that local chiefs would, or were entitled to, claim that their territorial rights had been violated. In 1841 he confirmed that his Act of 24 August 1839 authorised British officers to search and detain slavers found at anchor off ports in Portuguese Africa and in Portuguese African waters: where there were Portuguese authorities in the vicinity prior permission should be sought but, even if it were refused, an officer should not be deterred from doing his duty provided that he did not expose his ship to attack from Portuguese shore batteries.

Throughout the period 1839–44 the British government persisted in their efforts, which already went back more than a decade, to strengthen the Anglo-Brazilian anti-slave trade treaty of 1826. In 1839 it had seemed as though the stalemate which had arisen from the refusal of the Brazilian Chamber of Deputies to ratify the additional equipment and break-up articles signed by Henry Fox and Manuel Alves Branco in July 1835, and vital to any effective British preventive system, had at last been broken: British commissary judges in Rio de Janeiro and Freetown, eventually supported by the British government, had reinterpreted the treaty as it stood and argued that British warships already had the right to search and capture Brazilian vessels suspected of intending to trade in slaves. Successive Brazilian governments, however, refused to accept the validity of the British reinterpretation of the treaty. As a result, disputes over the legality of British captures were endemic in both the Anglo-Brazilian mixed commissions, and British naval officers could never rely with any confidence on the condemnation of their prizes. It was a thoroughly unsatisfactory state of affairs and, while ever the British government was reluctant to resort to the kind of measures which had been taken against the Portuguese slave trade (including the adjudication of prizes by British admiralty courts), one which could only be remedied by agreement with Brazil.

On 26 November 1826, a few days after the negotiations for an Anglo-Brazilian abolition treaty had been brought to a successful conclusion, Robert Gordon, the British minister in Rio de Janeiro, warned George Canning, the British Foreign Secretary, that the treaty was bound to be ‘in the highest degree unpopular’ in Brazil; it had been, he later admitted, ‘ceded at our request in opposition to the views and wishes of the whole Empire’. It was, however, six months before the Brazilian Chamber of Deputies was given an opportunity to voice the strongly anti-abolitionist opinions which the vast majority of influential white Brazilians undoubtedly held. In May 1827 the Chamber reassembled after a recess lasting almost twelve months and on 22 May the new Brazilian Foreign Minister, Marquês de Queluz (João Severiano Maciel da Costa)—himself the author of an abolitionist tract, Memoria sobre a necessidade de abolir a introducção dos escravos africanos no Brazil (published in Coimbra in 1821), in which he had argued that the trade would nevertheless be necessary for some little time to come—presented a report on the recent treaty negotiations with Britain in which he maintained that the Brazilian government had been forced to sign the treaty of 23 November 1826 entirely against their will. On 16 June 1827, the treaty was put in the hands of the Chamber's Comissão de Diplomacia which narrowly, by three votes to two, agreed to accept it.

The final suppression of the Brazilian slave trade during the years 1850–1—twenty years after it had been declared illegal by treaty with Britain and more than forty years after Britain had abolished her own share of the transatlantic trade and made her first official abolitionist overtures in Lisbon and Rio de Janeiro—did not immediately remove the slave trade question from Anglo-Brazilian relations. On the contrary, both the memory and the legacy of a conflict so protracted and at times so bitter poisoned relations between the two countries for many years to come. And serving most effectively to keep the slave trade controversy alive was the continued existence of the Aberdeen Act long after the trade had been abolished.

At the time of its passage in 1845, Lord Aberdeen had looked upon his Act, like the Palmerston Act before it, as an exceptional, temporary measure; it would be repealed, he had indicated, either when Brazil signed an effective anti-slave treaty with Britain—as Portugal had done in 1842—or when Brazil co-operated with Britain and herself abolished the trade. Towards the end of 1851, believing that by their actions they had now clearly demonstrated their desire and their ability to put down the slave trade and to prevent its revival, the Brazilian government made a second attempt—the first had been made, prematurely, in October–November 1850—to persuade Britain to repeal the Act which had always been deeply resented in Brazil or, at the very least, not to enforce it in Brazilian inland and territorial waters.

In a letter to Peel on 18 October 1844, Lord Aberdeen forecast that Britain's relations with Brazil would soon become ‘unpleasant and complicated’. Partly as a demonstration of her independence from Britain, Brazil had insisted—as she had the right to do—on terminating the Anglo-Brazilian commercial treaty of 1827, one of the two treaties which had been imposed upon her more than fifteen years earlier as the price of British recognition of her independence from Portugal. The other treaty, the Anglo-Brazilian abolition treaty of 1826, was of indefinite duration and could not therefore be terminated unilaterally by Brazil. However, the treaty of 1817, which formed part of the treaty of 1826—and a crucial part, since it was under this treaty that the British navy exercised the right of search and the Anglo-Brazilian mixed commissions adjudicated upon captured Brazilian vessels—was not permanent. Indeed, it would have been brought to an end as early as March 1830 when the Brazilian slave trade first became entirely illegal had not Lord Palmerston taken advantage of the separate article of 11 September 1817 which permitted it to continue in force for a further fifteen years.

As the illegal slave trade to Brazil continued to expand throughout the late forties in the absence of any really effective preventive action by the Brazilian authorities and in defiance of the most extreme measures so far adopted against it by the British navy, opposition in England to what John Bright called Lord Palmerston's ‘benevolent crotchet for patrolling the coasts of Africa and Brazil’ gathered considerable momentum both inside and outside Parliament. It was towards the end of the previous decade that a degree of dissatisfaction with Britain's traditional anti-slave trade policies had first become apparent. Considerably strengthened and extended the British preventive system had nevertheless been given a further period in which to prove itself. If, however, its raison d'être was the suppression of the illegal transatlantic slave trade, or at the very least a substantial reduction of it, with the Cuban trade as well as the Brazilian still flourishing its failure could no longer be denied and there was evidence of a growing conviction that it ought now therefore to be dismantled.

Since its foundation in 1839 the British and Foreign Anti-Slavery Society, very much Quaker dominated, had been opposed to the use of armed force for the suppression of the slave trade and, consequently, completely out of sympathy with the anti-slave trade efforts of successive British governments, both Whig and Tory.