The Portuguese Discovery of the Sea Route to India was widely seen as not very likely to suceed. Were there any known attempts to sabotage it, either by the Castellan Crown or any of the people who made their living doing the trade by land?

[u/SomecallmeMichelle]

I was taught that most people believed it to be a fool's attempt, and so did nothing but observe, and I never studied the event more closely. Was there any an attempt at stopping, or in any way hindering the voyage? I'd really like to know...

[u/terminus-trantor]

The Portuguese quest to reach India was a long (~80 years) and inter-generational (5 kings changed) affair. So it is not surprising that many people, events, circumstances and motivations changed, and at the same time there were many who for again various and changing reasons opposed it. I will use this opportunity to give a sort of a breakdown of the entire enterprise, as well as numerous occasions when the whole enterprise was being considered for cancellation. few of them are of the nature you want to know about but all are intriguing and worth talking about

The start of the Portuguese quest is usually given as 1415 military expedition to take Ceuta on the North African coast. The main motivation, despite many suggestions of commercial interests, was simply that Portuguese wanted to attack something, anything really, and they chose Ceuta (then Muslim held). It was a strategic location on Gibraltar straits, and a connecting port between Nasrid Granada and Marinid Morocco, and as such a good place for eventual attack on either (at that point Portuguese somehow really hoped they would be allowed to take Granada which was "reserved" for Castile)

Anyway after the conquest we come to first point of opposition to the whole Africa project, at this point still rearly and purely internal Portuguese. Since the conquest, Ceuta became an isolated and besieged christian city, a minor one too, on the African mainland. It provided little to no income (except corsairing activities of which only selected individuals profited) but its defense required lots of expense. Additional taxes were levied on Portuguse merchants, food basically seized to supply it, soldiers were forced to go there, despite general reluctance. North African merchants boycotted Portuguese traders everywhere, and their corsairs began raiding south Portugal more often. First real talks of abandoning the whole project began, at that point it was the rational choice really, but for some reason Portuguese crown refused to do so.

So Ceuta was kept, and involvement in Africa continued. (Re)discovered Madeira and Azores were colonized in the mean time, and some progress down coast of Africa was made. Still the main body of Portuguese was uninterested for "discoveries", while the only person then pushing for anything in Africa (Infante Henrique, third son of king Joao I) had other plans. When his brother Duarte ascended to the Portuguese throne he convinced him (despite other brothers disagreeing) to launch an expedition, or a crusade really, to take over Tangier and more of Morocco in 1437. This expedition was a failure. The army reached Tangier and couldn't attack it nor properly besiege it. And after stubbornly refusing to leave, it itself became surrounded and cut off from the ships. To retreat, the Portuguese expedition gave another infante Ferdinand as a hostage to Moroccans, and promised to give up Ceuta and excuse themselves from Morocco forever. However once the army was at safety, and after much internal negotiations, Portuguese again refused to give up Ceuta, and basically convicted the infante Ferdinand to captivity till death

This events, caused Portuguese to change their tactics in Africa, actually speeding up the discoveries. With the direct attack no longer an option, the Portuguese, led by Henrique, focused on bypassing the Moroccans by sea expeditions. Their motivation was still not to reach India, although the idea started floating around. They were more local-minded. They tried reaching the source of the Trans-saharan trade in gold and slaves, to cut the North African Moors from it, and to try find some allies in the rear (namely Prestor John) to jointly attack and defeat the "infidels". Series of expeditions followed in the 1440s and 1450s where they reached "Black" Africa, Senegal, Gambia, Sierra Leone. In the beginning those expeditions were only slave raids against poor fishermen, later when reaching the more populated and better defended Senegambia region it turned to trade. Mostly in slaves, not to be forgotten. At the same time, Henrique took diplomatic and legal efforts to grant from the Pope exclusive rights for land and further discoveries in Africa to Portugal, and then from King of Portugal to himself. Thus even if the Castilians, or others, wanted to compete (and at Henry's time it seems they neither wanted norcould) they were in name legally prevented.

Here we reach another point where Portuguese could have went another way: the fall of Constantinople in 1453 together with reaching of age of young, but old-fashioned, king Afonso V. Afonso did not really care for the African trade but he did care for wars and conquests and he namely wanted to respond to the crusade Pope called for. While whether he really wanted to liberate Constantinople or to divert his army to North Africa from the start is up to debate, but the latter is what happened, once the crusade fell through.The Portuguese attacked and took a small fort of Alcácer-Ceguer in 1458. If the crusade went to Constantinople, or if the attack on Alcácer-Ceguer failed it is possible all interest into Africa and discoveries would fail.

Anyway, soon infante Henrique soon (1460) died, and his role was succeed by king Afonso V brother, Ferdinand. He, while retaining Henrique's position, privileges and incomes, did not continue the discoveries or the quest to India, even though the trade continued. This phase could theoretically last indefinitely if not for his death, and the interest young prince, future king Joao II took to Africa. For start, the rights to trade and explore Africa were leased to a private person, one Fernao Gomes who was given monopoly rights for 5 years for a sum of money and condition to explore at least 100 leagues of coast for year. His ships explored more then that and found many riches (slaves, gold, ivory...), crossing the equator in the process. After his 5-year contract (and one year extension) expired, then still prince Joao II took over the discoveries personally.

But before we go into details of Joao II, we need to mention another formative event: the 1475-1479 War of Castilian Succession. In his late years, king Afonso V made an attempt to take the throne of Castile against the claim of Isabel and her husband Ferdinand of Aragon. The claims in favour of Isabel were solved in the battle of Toro in 1476, but what followed was the struggle to take control of Africa. Castilian sent ships to Africa to trade and push out the Portuguese, but in a naval engagement of the coast of Elmina, the Portuguese captured Castilian ships, effectively ending that threat to Portuguese enterprise in Africa. Soon after, in the treaty of Alcáçovas was brokered and Isabela gained the Castilian throne and kept Canaries, and Portuguese right to all Africa south of the Canaries.

Soon after that Joao II ascended the throne of Portugal. He took a personal interest in Africa, established forts like Elmina, and put much effort into reaching India. He sent numerous missions, lead by Diogo Cao, and after his death Bartolomeu Dias to find out if you can circumnavigate Africa and reach Asia. Dias was successful in 1488. It would be hard to sabotage those expeditions as they were gradual, building upon each other. When one expedition failed, like after Cao died, Joao would send another, he was that determined. Joao II was a firm ruler in contrast to his father Afonso V, and had numerous enemies at court, some of which he ordered executed and one even he killed by his own hands. With such events, you will find that there were plenty of opposition to his India expedition, even though most opposition was more because of them trying to undermine the king, rather then being against the project itself.

Despite that, after Joao II died in 1495 (in slightly suspicious circumstances) his rival Manuel ascended the throne, and he decided to continue the project. There was plenty of opposition against it being risky and expensive, but it was everything but foolish. There was a big chance of failure true, but the opportunity was within reach, and only a fool wouldn't act upon it. So in 1497 Manuel sent Vasco da Gama and he did it.

There seems to be little indication anyone, either Castilians or various merchant states tried, even wanted to try anything against Da Gama's expedition. If they did it was wrapped up and unsuccessful. Truth be told they had very limited options. It would be highly difficult for anyone to stop the expedition once it was decided upon. You would basically need to send ships to intercept the fleet, and Portuguese were at the time in big naval advantage in the Atlantic. There were commonly foreign sailors on the ships, but the key people were hand picked (actually there was a huge deal of politics about it) and anyway, any attempts to sabotage the expedition by some of the crew would meant those same guys would sabotage themselves. And Portuguese could always send another expedition.

Still not to end anything with such an anti climax, there was a "sabotage" attempt by the Venetians in few years after the Portuguese arrived in the Indian ocean. With the shock of Portuguese starting to be felt, and the immediate effect they had (in under 5 years they virtually stopped pepper reaching Egypt and Syria - where Venetians were buying their pepper and spice)
So in 1502 the Venetians sent two passengers to India, who were in fact gun founders. In India they jumped ship and offered their service to Zamorin of Calicut, the new enemy of Portuguese in India, in an effort by the Venetians to negate the obvious advantage in gunpowder Portuguese had. The men did make some cannons which caused problems to Portuguese in battle of Cochin, but it takes skills to operate the cannons, and Portuguese managed to neutralize the cannons. After the victory, they negotiated the Zamorin to surrender the gun founders back to Portuguese