Globally, Spain remained an important naval and military power, depending on critical sea lanes stretching from Spain through the Caribbean and South America, and westwards towards Manila and the Far East. The 18th century saw an ongoing struggle between the growing naval power of the rising imperial power Great Britain and Spain that worked to maintain it transoceanic links with its overseas empire, still by far the largest of the time. The number of Spanish galleons deploying across the Atlantic sea routes increased significantly in the first half of the century, undoing the decline of the latter 17th century. Britain used the exclusive right to trade in Spanish America for thirty years which she had gained at the end of the War of the Spanish Succession as an casus belli military intervention, such as during the War of Jenkins' Ear (1739–1748). During the Seven Years' War (1756–1763), Britain attempted to leverage its existing island bases along the Spanish Main and the Spanish West Indies, capturing Havana and Manila, but in each case practical and strategic considerations led to their return in exchange for Florida. During the American War of Independence, Spanish forces reconquered Florida and assisted the American rebels with arms and soldiers and by attacks on British trade and supplies. Both Spain and Britain made extensive use of privateers throughout the war, the Spanish fully exploiting the British aversion to using the convoy system to protect its expensive merchant assets in times of war. The earlier War of the Polish Succession was still seen as positive for Spain, as the kingdom did recover the territories lost after the war of Spanish succession, in Italy. However, during the Seven Years' War, three Spanish attempts to conquer Portugal ended in crushing disasters.