I don't know who has read the book by Orson Scott Card, Pastwatch: The Redemption of Christopher Columbus, but the book details an alternate history where Columbus takes up the life-goal of rescuing Constantinople from the Turks. The book aside, what if this was true?
Let's say in August 1476, during his encounter with the French corsair Guillaume de Casenove, Columbus prays to God for rescue, promising to lead a crusade to re-take Constantinople in return. After surviving, he sets out to fulfill this covenant. He skips out on his voyage to Iceland, staying in Lisbon, marrying the daughter of that Genoese nobleman to make a name for himself. But by 1482 he's ready, having spoken as a supporter of a new crusade to all he's come across that might be interested in investing. He visits the court of the newly-coronated King John II of Portugal, with no success, he is too busy with internal matters. A year later he would attempt to pull Spain into a new crusade with no success: Isabella and Ferdinand wouldn't focus on any new wars while the Reconquista was still on.
Columbus determined that to finance the crusade he would have to get papal support and the passion of the Italian states. Meanwhile he was fund-raising and gaining new investors. In 1484 he arrives in Rome just as Pope Innocent VIII succeeds Pope Sixtus IV and makes his speeches supporting a crusade against the growing Ottoman Empire. Columbus sees this as a sign and immediately requests an audience with the Pope, making it known that as an independent fund-raiser and investor he has already gathered money to put together a small army and navy to send against the Turks.
By 1485 the Pope receives him and in a famous meeting blesses Christopher Columbus as a holy warrior of God. Columbus tells Pope Innocent VIII that the recent deliverance of Prince Cem, pretender to the Ottoman throne, to his custody is a godsend. He convinces Pope Innocent that he has been ordained by God to lead a Tenth Crusade. At that moment Innocent VIII becomes very enthusiastic at the proposition of such a champion. However, he is currently plotting with noblemen in the Kingdom of Naples to overthrow Ferdinand I, who had led the kingdom against the Papal States in the War of Ferrara in 1482. He thus enlists Christopher Columbus to immediately use his funds and put together a small army to support the coup.
In the later part of the year, Columbus fulfills his end of the bargain, landing six ships and a small army at Naples when signalled, and this small force led by Columbus himself, determined that he has God's protection, is enough to turn the tide of the battle. Ferdinand II is imprisoned and sent to the Vatican, while the Francesca Coppola becomes the new King of Naples in a close alliance with the Papal States. Coppola puts Christopher Columbus into a high position in the Naples navy, with his expenditures for his support in the coup paid twice over. With King Francesca pledging his support for a new crusade (perhaps to gain territory in Greece or at least open them up to trade once more), Pope Innocent VIII began rallying the European monarchs once more to embark in a war against the Ottoman Empire. He would find a fiery passion in the new French King Charles VIII, who had just taken the throne, as well as in the Venetian Republic, who had already suffered from Ottoman aggression and felt the need to secure their possessions by inflicting damage.
What do you guys think? Realistic so far? Any ideas on how to make this work and what might result from all of this?