Magellan’s expedition 1518 – 1522 circumnavigated the globe via many places including Cebu Philippines. Although Magellan was a Portuguese, he was under contract to Spain who had funded his trip on the basis of his crucial advice that he could approach the Spice Islands from a Westerly direction and that there were many islands discovered by Portugal that lay within the Spanish sphere.
However only a few years after his return, in 1529 Charles signed the Treaty of Zaragoza which agreed a new demarcation line at 297 leagues east of Malacca which would divide Spanish territories to the East and Portuguese territories to the West and dictate title to any newly discovered lands accordingly. Charles essentially contracted out of any further claim to sovereignty of the Moluccas (which included Philippines) for the sum of 350,000 ducats.
Why this irrational behaviour? There are many likely factors…..
Was it because Charles was made aware that the Portuguese had already reached the Spice islands as early as 1512 and that this was unbeknown to him at the time of striking his deal with Magellan? If so he was conceding to Portugal that he had lost the race for the Spice Islands and that Magellan had effectively come back empty handed for Spain.
Was it because he was unaware or even mislead about the geographical position of unexplored territories in the west of the Pacific? He certainly knew well that he was at a tactical disadvantage to Portugal due to the much longer route from the West as compared to the from the East. Its therefore likely he opted for the certainty of a demarcation line in the mistaken belief that there were other undiscovered land masses to which to he could claim title.
Was it because he was desperate for the cash? Not as strange as it sounds; the Spanish crown was formally declared bankrupt in 1557, 1560, 1569, 1575 and 1596.
Was it because Charles was confident that he would in time, be able to prove that Philippines fell within the Spanish meridian as stipulated in the treaty of Zaragoza in which case he would have to repay the money to re-claim the territory? Of course he was never able to prove this.
Was it because the Portuguese were able to convince the arbiter of the treaty (the Pope) that they were already established in the region and were successfully evangelising the local poulations there?
Was it because since getting married in 1525 his actions were heavily influenced by his wife and family? She was the sister of John 3rd of Portugal. Also John 3rd was married at the same time to Charles sister therefore Charles was heavily outnumbered by his in-laws. This would seem to be a key factor.