So long as Brazilian governments proved unable or unwilling to enforce their own legislation prohibiting the importation of slaves into Brazil in the period after 1830, Britain, or to be more precise the British navy, represented the only serious threat to the continued existence of the illegal Brazilian slave trade. And the navy's success or failure in containing the Brazilian slave trade—it was doubtful whether, without the co-operation of Brazil, the British navy could ever completely suppress it—was not dependent solely on the number of warships available for anti-slave trade duties. It was also, and more immediately, dependent on the extent to which the necessary powers to visit, search and capture slave vessels in the Brazilian trade had been accorded British warships under the various anti-slave trade treaties which Britain had signed with Brazil and other foreign powers.

Considering the attention Canning had devoted to the precise nature of the treaty Britain had extracted from Brazil in 1826, it is remarkable how little thought he seems to have given to the crucial question of how the Brazilian slave trade, once illegal, could be suppressed by the British navy if, as seemed likely even at the time, the Brazilian government failed to fulfil its treaty obligations. For three years (1827–30) the Brazilian trade remained illegal only north of the equator and the treaty of 1826 provided that, during this period, British cruisers should continue to operate under the Anglo-Portuguese Right of Search Convention of 1817; an Act of 2 July 1827 (7 and 8 Geo. IV cap. 74) authorised Anglo-Brazilian mixed commissions, which were to sit in Rio de Janeiro and Sierra Leone, to adjudicate any Brazilian ships captured north of the line—provided they were carrying slaves.

By the late 1830s the failure of Britain's efforts to contain, much less to suppress, the illegal transatlantic slave trade—the Cuban trade as well as the Brazilian continued to flourish—was causing increasing concern amongst English abolitionists. Many were coming round to the view, frequently expressed by diplomats, commissioners and naval officers directly concerned with improving and implementing the anti-slave trade treaties, that Britain would soon have to resort to more drastic measures, if necessary without the consent of the three major offenders, Portugal, Spain and Brazil. Lord Brougham, for instance, addressing the House of Lords in January 1838, expressed the hope that the final suppression of the trade would be the earliest and most enduring glory of Queen Victoria's reign. For how much longer, he asked, with heavy irony, would Britain continue to ‘pause and falter, and blanch and quail before the ancient and consecrated monarchy of Brazil, the awful might of Portugal, the compact, consolidated, overwhelming power of Spain' while Africa was ravaged? When on 10 May 1838, for the first time in three years, the Commons debated the slave trade question, a motion demanding that greater pressure should be brought to bear on foreign powers, and especially on Portugal, found support on both sides of the House and was carried without a division.

The independence of Brazil was essentially the result of a hopeless attempt by Portugal during the years 1821–2, following Dom João's return to Lisbon after an absence of thirteen years, to put back the clock and reduce Brazil, politically and economically, to its former colonial status. Brazil had progressed too far since the flight of the Portuguese court to Rio de Janeiro in 1807–8 for anything less than complete political and economic equality with the mother country to be any longer acceptable: Brazilian ports had been open to world, and especially British, trade, and the influx of ‘new people, new capital, new ideas’ had stimulated economic development and modernisation; Brazil had been governed from Rio de Janeiro, not Lisbon—indeed since December 1815 its status had been that of a kingdom equal with Portugal; the development of Brazilian self-consciousness had been considerably accelerated. When, therefore, Portugal proved intransigent in her demand for Brazil's capitulation to Portuguese rule, many Brazilians—and some Portuguese whose roots and interests were now in Brazil—saw no alternative to secession from the Portuguese empire. Left behind in Rio de Janeiro as Prince Regent, Dom João's eldest son, the twenty-four-year-old Dom Pedro, choosing to lead rather than be overwhelmed by the growing movement for independence, threw in his lot with the Brazilians and, when ordered to do so, refused to return to Portugal.

For 300 years, from the beginning of the sixteenth to the beginning of the nineteenth century, the transatlantic slave trade—the forced migration of Africans to work as slaves on the plantations and in the mines of British, French, Spanish, Portuguese and Dutch colonies in North and South America and the Caribbean—was carried on, legally, and on an ever-increasing scale, by the merchants of most Western European countries and their colonial counterparts, aided and abetted by African middlemen. Indeed, until the second half of the eighteenth century, when (at a conservative estimate) 70–75,000 slaves were being transported annually across the Atlantic, scarcely a voice was raised against it. On 25 March 1807, however, after a lengthy and bitter struggle, inside and outside Parliament, it was declared illegal for British subjects (and at this point during the Napoleonic Wars at least half the trade was in British hands) to trade in slaves after 1 May 1808: opposition to the slave trade on moral and intellectual grounds had gathered momentum during the preceding twenty years and changing economic conditions, which to some extent reduced the importance to the British economy of the West Indian colonies, for whom the slave trade was a major lifeline, while at the same time creating new interest groups unconnected with and even hostile to them, had greatly facilitated its abolition.