From Seven Myths of the Spanish Conquest, by Matthew Restall (Oxford, 2004), Kindle Loc. 348-399:
Columbus had profound Portuguese connections. Although he was Genoese and the sponsor of his voyages across the Atlantic was Queen Isabella of Castile, Columbus spent much of his life from the 1470s on in Portugal. In the late 1470s he married the daughter of a Portuguese Atlantic colonist, and he repeatedly sought royal Portuguese patronage before and after first approaching the Castilian monarch….
This context is so important because it is by looking at Portugal before and during Columbus’s years there that one can see the degree to which the transplanted Genoese navigator had neither a a unique plan nor a unique vision nor a unique pattern of previous experience. Many others created and contributed to the expansion process of which Columbus became a part. Beginning 200 years before Columbus crossed the Atlantic, southern European shipping broke out of the Mediterranean into the Atlantic. The Vivaldi brothers, most notably, set off from Genoa in 1291 on what turned out to be a one-way voyage west across the Atlantic. Then, in the fourteenth and early fifteenth centuries a new zone of navigation was created that was bordered by the Azores in the north, the Canary Islands in the south, and the Iberian-African coasts in the east.
Finally, from the 1420s on, a further stretch of exploration and navigation into the mid- and west Atlantic was created and charted. In the 1450s and 1460s, Flores, Corvo, the Cape Verde Islands, and the islands of the Gulf of Guinea were explored. The Madeiras and Canaries were settled and turned into sugar-plantation colonies and by 1478 the former was the largest sugar producer in the Western world. Maps of the time show how important and extensive was the discovery of Atlantic space; speculation about the lands and features of the ocean was the most noteworthy feature of fifteenth-century cartography.
Although men from Italian city-states were involved from the start, and Castilians increasingly participated in the process (especially, from the late-fourteenth century on, in hostile competition for control of the Canaries), it was Portugal that dominated this expansion. Italian navigators were systematically and most effectively co-opted by the Portuguese monarchy (later joined by the Flemish), permitting the new Portuguese empire to control Atlantic settlement (except for the Canaries) and the agenda of expansion….
Columbus tried to become part of this process with growing desperation in the 1480s and 1490s. He failed for so long because he lacked the connections and persuasive ideas of other navigators. Even after he succeeded in crossing the Atlantic and returning, the extent of his success was questioned and questionable within the context of the time. The islands he had found (in the Caribbean) fell within the zone assigned to the Portuguese by the 1486 papal bull. And although in 1494 the papacy brokered a Portuguese-Castilian treaty that redefined these zones, it became increasingly apparent during the 1490s that Columbus had not found the much-sought sea route to the East Indies—but had been lying about it to Queen Isabella. Then, in 1499, Vasco da Gama returned from his successful voyage around the Cape and it became clear that the Portuguese had won the competition after all.
Columbus’s career was irreversibly damaged. His claim to have found islands off the coast of Asia, and thus the coveted sea route to that continent, rang hollow in the face of mounting evidence that these were new lands entirely. Columbus seemed to be lying for the sake of his contractual rewards. Perceiving the extent of his failure and his duplicity, the Castilian crown dispatched an agent to the Caribbean to arrest Columbus and bring him back to Spain in chains. Although he was later permitted to cross the Atlantic, he was forbidden to revisit the Caribbean and was stripped of the titles of Admiral and Viceroy of the Indies—titles he had fought to be included in his original contract and arguably the chief goal of his career. Meanwhile, those titles were conferred by the Portuguese crown upon da Gama.
The fact that it was Columbus’s voyages, not da Gama’s, that would lead to the changing of world history was not to the Genoese’s credit. His discoveries were an accidental geographical byproduct of Portuguese expansion two centuries old, of Portuguese-Castilian competition for Atlantic control a century old, and of Portuguese-Castilian competition for a sea route to India older than Columbus himself. Furthermore, had Columbus not reached the Americas, any one of numerous other navigators would have done so within a decade. Most obviously, the Portuguese Pedro Álvares Cabral explored the Brazilian coast in 1500, likewise arriving there in an attempt to reach Asia (by rounding the Cape).