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Ernest was named Governor and heir of the Spanish Netherlands by his uncle Philip II of Spain in 1576 (instead of don Juan of Austria becoming Governor) and - being a) able to count on Austrian troops other than the Spanish ones and b) had more independence from Madrid and was able to make promises for the future after Philip - was able to pacify the whole Netherlands, preventing the birth of the OTL Dutch Republic. Philip was not exactly happy about a lot of Ernest’s promises but had already more-or-less gifted the Netherlands to his nephew - with the only conditions to choose Ernest’s bride and who Netherlands would go back to Spain if Ernest’s line ended - and in the end he must recognize who the methods of Ernest (and of Alessandro Farnese, who Philip had sent there as military commander) were pretty effective.
Philip had taken long time before offering any bride to Ernest, until he offered his own eldest daughter, infanta Isabella Clara Eugenia, as bride in 1583. The 17 years old infanta was formerly engaged - until few days before being offered to Ernest - to his older brother, Holy Roman Emperor Rudolf II. As Rudolf was delaying the wedding to Isabella, he and Philip had a fight over it and in the end Philip decided who Archduchess and future Duchess of Burgundy now was better than maybe Holy Roman Empress sometime in future so he broken the engagement of his daughter to Rudolf and offered her to Ernest, who happily married her.
As Rudolf never married and Ernest became Emperor after his death in 1612, Philip’s decision in the end was the best one and if the King of Spain had died almost fifteen years before Rudolf, the unmarried state of the latter had always made Philip quite hopeful in a future succession of his grandson as Holy Roman Emperor.

Considering who none of Ernest‘s surviving brothers (mad Emperor Rudolf was unmarried, Archduke of Further Austria Matthias had no child by his wife Anna of Tyrol (another former fiancé of Rudolf the Mad), and the other three were unmarried for their ecclesiastical status: King Maximilian I of Poland, Grandmaster of the Teutonic Order; Cardinal Albert, Viceroy of Portugal; Archduke Wenceslaus, Grand Prior of the Order of Malta in Castile who died at only 17 years old) had children we can say who Philip’s decision to marry Isabella to Ernest saved the main line of the Austrian Habsburg and likely the whole Holy Roman Empire as the man who would have ruled Austria if the male line of Maximilian II and Maria was extinct was the ultra Catholic Ferdinand, Archduke of Inner Austria, whose religious positions had made him a lot of enemies among the Protestants who instead had a quite good relations with the Catholics but moderate Emperors Ernest I and Maximilian III and with King Ferdinand of Poland and Hungary.

As Ernest‘s brother, King Maximilian I of Poland, had opted to keep his ecclesiastic status and role as Grandmaster of the Teutonic Order after his election in Poland, Ernest started to suggest pretty early his second son Ferdinand as potential successor for Maximilian in Poland and after securing his election as future King of Poland had Ferdinand also elected as his own successor in Hungary, with his eldest son Maximilian elected as King of the Romans - title of the elected successor of Holy Roman Emperor - and as future King of Bohemia.