British from South Africa rather than Portuguese. The other major force for change was the rivalries that developed between European nations in the century between the end of the Napoleonic Wars and the outbreak of World War I. Forbidden from fighting each other by the "balance of power" established by the Treaty of Vienna, they competed in other ways including scientific discoveries, athletic competitions, exploration and proxy wars. Although not a major power anymore, Portugal participated in the competition, especially by sending out explorers to solidify their claim to all of the land between Angola and Mozambique. That bought them into conflict with men like Cecil Rhodes, whose own vision of an empire from "Cape to Cairo" required that the British gain control over the same land (see British Ultimatum). European rivalries appeared most often as commercial competition, and in 19th century Africa, that included the right to move goods by steamboat along rivers. The British had a head start thanks to their early adoption of steam technology and their supremacy on the high seas. They became the strongest proponents of the principle of "free trade" which prohibited countries from creating legal barriers to another country's merchants. Occasionally, Portuguese leaders resisted, but the British alliance provided sufficient benefits to convince various administrations to go along (although they faced revolts at home and in their colonies). It was Portugal's claim to the land on either side of the mouth of the Congo River that triggered the events leading up to the Congress of Berlin. That claim, which dated from Diogo Cão's voyage in 1484, gave Portugal places from which naval patrols could control access to Africa's largest river system. The British eyed this arrangement with suspicion for years, but paid tariffs (like everyone else) for the right to trade there, mostly for slaves. After the abolition of slavery got underway, the Portuguese dragged their heels, so in 1839 the British government declared its right to inspect Portuguese ships for evidence of slave trading with or without Portuguese consent. That stirred the Portuguese to action, and in a subsequent series of agreements made in the 1840s, the British acquired the right to land their ships to land where no Portuguese authorities were present. When the Portuguese refused to renew the agreement in 1853, the British ceased paying tariffs at the ports on either side of the Congo River mouth, claiming that Portugal's