At the same time, if an attempt to "terminate with extreme prejudice" failed and Magellan reached his target, some form of negotiation would follow.
Any new line drawn in the east would cover territory that had not yet been "discovered". There was a vast stretch of unknown and uncharted territory to the south of the Portuguese outposts in Timor and the Moluccas in the area beyond Marco Polo's "Great Java".
So there was a flurry of Portuguese immediately after news of Magellan's expedition leaked out.
By the time Magellan's five ships departed from Seville on 10 August 1519, the Portuguese had dispatched two squadrons to the East Indies. Three caravels under Rafael Catanho set out at the end of 1519.
Another fleet under Jorge de Brito left later.
Proponents of the Portuguese priority in the "discovery" of Australia focus on Catanho's three caravels.
When they arrived at the Portuguese headquarters in Goa, Catanho delivered the Portuguese king's instructions for an expedition to search for the Isles of Gold to governor Diogo Lopes de Sequeira.
However, Catanho would not command the expedition. The position went to Cristovao de Mendonca, with Pedro Eanes as second in command.
Mendonca's three caravels left Goa bound for Malacca and points beyond but stopped off to help reinforce a Portuguese stronghold in northern Sumatra along the way.
What happened after the vessels arrived in Malacca is, predictably, shrouded in secrecy. Any details would presumably have found their way back to Lisbon.
However, everything relating to the Portuguese exploration stored in Lisbon's Casa da India e Mina, were buried in the rubble or went up in flames in the great earthquake that struck the Portuguese capital on 1 November 1755
However, we know that Mendonca and at least one ship returned to Goa. He went on to serve as the Portuguese Captain-Major in Hormuz and died there in 1532.
Various hypotheses attempt to fill in the missing details of his voyage. Most of them relate to the so-called Dieppe maps produced by French cartographers, presumably from Portuguese sources, later in the 16th century.
Those matters are explored thoroughly in The Portuguese Question, which attempts to build a case for some form of Portuguese activity in Australian waters in the early 1520s.