Maximilian started the vast Habsburg Empire by a well balanced marriage policy and to be honest a big portion of luck, for instance when all the heirs of the Spanish crown died leaving only Joanna of Castilia to be heir of the Spanish Crown together with her husband the Habsburg Philip the Handsome

https://en.wikipedia.org/wiki/Philip_I_of_Castile
"The son of Holy Roman Emperor Maximilian I by his first wife Mary, Philip was less than four years old when his mother died, and upon her death, he inherited the greater part of the Duchy of Burgundy and the Burgundian Netherlands as Philip IV. In 1496, his father arranged for him to marry Joanna of Castile, second daughter of Ferdinand and Isabella, rulers of Aragon and Castile respectively. Around the same time, Philip's sister Margaret was given in marriage to Joanna's brother John, as part of an agreement between their fathers. Within four years after the wedding, Joanna became heir presumptive to Aragon and Castile, following the deaths of her brother, elder sister and infant nephew during that period. In 1504, aged 27, Philip became king of Castile jure uxoris when his mother-in-law died and Joanna succeeded her. He died only two years later, leaving his wife distraught with grief.
 
Marriage policy that he started with the Burgundy of Mary of Burgundy.
https://www.habsburger.net/en/chapter/marrying-burgundy-maximilian-i-and-mary-burgundy

And then with his son from Mary of Burgundy: Philip the Hansome to Juana La Loca
https://www.habsburger.net/en/chapter/marrying-spain-philip-fair-and-joan-mad

And with the marriage policy, while it was yet not enough, he got at last Bohemia and Austria
https://www.habsburger.net/en/chapter/marrying-bohemia-and-hungary-double-weviennadding-

But to be fair I wanted to add that there was a big part of luck and it was not only by marrying. For instance his marriage with Mary of Burgundy was already prearanged by her father Charles the Bold and when the French king tried to bring Mary in his sphere of influence, the States of the Low Countries put Mary nearly on arrest in Ghent till she chose for Maximilian, the states fearing for their economy and prefering Maximilian...
And I am glad that it is that well expressed on the site here mentioned:
https://www.habsburger.net/en/stories/tu-felix-austria-nube
Bella gerant alii, tu felix Austria nube – ‘Let others wage war: thou, happy Austria, marry’. Emperor Maximilian I is credited with the greatest success in the implementation of this policy – through his own marriage and those of his son Philip the Fair and his grandson Ferdinand, the dynasty gained Burgundy, Spain, Bohemia, and Hungary. However, the famous saying ignores the many strokes of luck that contributed to the rise of the Habsburgs – and also the fact that their gains were by no means always achieved without loss of blood. Wars were an equally important element of Habsburg policy as marriages.


And about the life of Maximilian
https://www.britannica.com/biography/Maximilian-I-Holy-Roman-emperor

Kind regards, Paul.