WI Juana of Castilla married Manuel of Portugal?

It is, but at the time Juana's first marriage was being determined, she had an older brother and sister and any issue they might produce ahead of her in the Aragonese and Castillian lines of succession. Not to mention that Ferdinand might well produce another son by a second marriage, leaving Juana a secondary heir to Castille-Leon alone.

Oh sure. But if you had to gamble, she is preferable to Brittany from the perspective of "surround France".

Brittany simply isn't strong enough to be a threat to France.

A useful foothold (especially for England), maybe, but not a threat.
 
It is, but at the time Juana's first marriage was being determined, she had an older brother and sister and any issue they might produce ahead of her in the Aragonese and Castillian lines of succession. Not to mention that Ferdinand might well produce another son by a second marriage, leaving Juana a secondary heir to Castille-Leon alone.

Well the double-marriage was intended to solidify their alliance and wasn't aimed at inheriting the lands of the other. That that happened was due to unlikely and unforeseen, yet not impossible events.
 
But what about the original question? :p

Could Portugal and Spain be united by a King Manuel-Queen Juana union? How would this affect the many unprecedented events of the period like the conquest of new empires in the Americas?
 
But what about the original question? :p

Could Portugal and Spain be united by a King Manuel-Queen Juana union? How would this affect the many unprecedented events of the period like the conquest of new empires in the Americas?

It would require any prior heirs to Juana dying off, to both Castile-Leon and to Aragon.
 
But she was the heir IOTL.

She's only the heir after those before her are dead.

http://fmg.ac/Projects/MedLands/ARAGON%20&%20CATALONIA.htm#_Toc203441670

King Fernando II & his first wife had six children:
1. Infanta doña ISABEL de Castilla y Aragón (Duenas 2 Oct 1470-Zaragoza 23 Aug 1498, bur Toledo). Susceptible to melancholy, probably inherited from her maternal grandmother, she experienced a period of profound depression and religious fervour after the death of her first husband. She was recognised as heir to the throne of Castile 29 Apr 1498, and declared Pss de Asturias. m firstly (by proxy Seville 18 Apr 1490, in person Lisbon 22 Nov 1490) Infante dom AFONSO de Portugal, son of JOÃO II King of Portugal & his wife Infanta dona Leonor de Portugal (Lisbon 18 May 1475-near Santarem 13 Jul 1491, bur Batalla). He died after falling from his horse. m secondly (Valencia de Alcántara 30 Sep 1497) as his first wife, MANOEL I King of Portugal, son of Infante dom FERNANDO de Portugal Duque de Viseu & his wife Infanta dona Brites de Portugal (Alcocheta, Tago 1 Jun 1469-Lisbon, Royal Palace of Rive 13 Dec 1521, bur Belem, Monastery of São Geronimo). Infanta Isabel & her second husband had one child:
a) Infante dom MIGUEL da Paz de Portugal (Zaragoza 23 Aug 1498-Gránada 19 Jul 1500, bur Zaragoza). Principe de Asturias from birth, as heir to the throne of Spain.



2. Infante don JUAN de Castilla y Aragón (Seville 30 Jun 1478-Salamanca 4 Oct 1497). Principe de Asturias. m (Burgos 3 Apr 1497) as her first husband, MARGARETA Adss of Austria, daughter of Emperor MAXIMILIAN I Archduke of Austria & his wife Marie Dss de Bourgogne (Brussels 10 Jan 1480-Mechelen 30 Nov 1530). Infante Juan & his wife had one child:
a) daughter (b and d Alcalá de Henares early 1498).



3. Infanta doña JUANA de Castilla y Aragón (Toledo 6 Nov 1479-Tordesillas 11 Apr 1555). Lived in Flanders with her husband. She replaced her nephew Miguel Infante de Portugal as heir to the throne on the former's death in 1500, returning to Spain briefly from Mar-Dec 1502. She succeeded her mother in 1504 as JUANA “la Loca” Queen of Castile, arriving in Spain 26 Apr 1506. Her father left the Government of Spain to her husband, retiring to Naples. Having shown signs of mental instability from an early age, Queen Juana descended into insanity after her husband's death, and was restrained at Tordesillas from 14 Sep 1509 for the rest of her life. She succeeded her father in 1516 as Queen of Aragon, but the Cortes accepted the succession of her son Carlos 7 Feb 1518, on condition of his abdication should his mother recover her reason. A brief insurgence in her favour took place Aug/Sep 1519, but it was crushed by King Carlos. m (Lille 20 Oct 1496) PHILIPP Archduke of Austria, son of Emperor MAXIMILIAN I Archduke of Austria & his wife Marie Dss de Bourgogne (Bruges 22 Jul 1478-Burgos 25 Sep 1506). He is sometimes referred to as FELIPE I King of Castile.
- KINGS of SPAIN.


King Fernando II & his second wife had one child:
7. Infante don JUAN de Aragón (b and Valladolid 3 May 1509). Principe de Gerona.
 
I think that still genetics could cause problems down the line, as Hapsburgs or no I think the Iberian Kingdoms did intermarry or at least the Trastamarias did.
 
I've been watching the TV series "Isabel" lately and on that note there is a priceless scene that basically goes "We need a papal bull to marry you to Fernando because you are second cousins. We first asked for it when you were 3 but the Pope refused. He did, however, approve the marriage of your half-brother Enrique to Juana of Avis who was also his cousin".

Mind you, at this point in the show Isabel has been courted by Afonso of Avis, brother of Juana (and therefore Isabel's cousin as well), and another marriage was proposed between Juana's daughter and Isabel's other brother Alfonso - his 'fiancée's' uncle. Nevermind that if you know your History you know that Afonso ultimately marries Juana Jr., his 4 times younger niece....
 
I think that still genetics could cause problems down the line, as Hapsburgs or no I think the Iberian Kingdoms did intermarry or at least the Trastamarias did.

In part that practice does seem to have been more common with the Spanish Habsburgs than with the Austrian Habsburgs. Besides the marriage practices of the houses of Bourbon, Savoy, Wittelsbach weren't that different.
Furthermore if you purely focus on the 'available gene pool', then the rReformation didn't help either.
 
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