Come on. This is modern Europe, not neolithic precolombian America.
You are talking about a 20 million people kingdom, in an age when manoeuvering armies numbered 20 to 30,000 soldiers at max.
Well, France had 20 million and Spain 8 million.
The military technics did not allowing such a conquest.
Italy had more people than Spain and Spain conquer the country...till the Pope had to see the Spanish - Imperial Power in Rome...The Spanish Power in Italy came from Sicily to Genoa or Lombardy... and you forgett Ottomans... conquered Greece, Bulgaria, Romania, Hungary, Yugoslavia, Albania, Egypt etc etc...
But in France, Spanish Armies were in North, South, East and West... from Paris (The Spanish Tercios were in Paris from 1590 to 1592)...to Normandy...
Winning battles does not mean conquering countries.
And Wars, my friend.. and wars...
And you are ignoring the french salic law. A french husband could become king of a foreign country. No foreign husband could become king of France. This law was unbreakable even in the worst moment of the religious wars. The catholic league (catholic extremists to put It quickly) prefered to have a priest of the royal family on the male line (the cardinal of Bourbon) on the throne than a foreigner.
But if you conquer the country, you can break the Salic Law, I think.
To unite kingdoms in those ages, the right solution was marriage, not war. Just consider the marriages between Tudors and Stuarts, or better what the Habsburgs did with Castile-Aragon and with Bohemia-Hungary.
You are right, the famous phrase
Alii bella gerunt, tu felix Austria nube. Both Austria - Hungary Empire and Spanish Low Countries came from marriages (and the union between Castilla and Aragon.. but not Navarre, conquered by the Catholic King to the Frenchmen). And not Italy.. The Spanish Dominions in Italy came from War. (Aragon/Spain).
You can Tix such a combination for Spain and France. But if you want It to be realistic, it needs to happen at the latest in the early 16th century. And it must happen either with french males or to happen at the latest in the early 14th century.
Yes about XVI Century. but not the rest of your phrase
Why French? Because are better than Spaniards? They didn´t show that in Pavia...or in Genoa...or in Saint Quentin...or in Garegliano or in Sicily or in Bicocca or in Landriano...No French.. or Spanish or nobody.. if The Spanish infantry beat the French Heavy Cavalry in Cerignola, the German Black strip mercenaries or the famous Swiss... Why a French? Not French. A Spanish Blodd Habsburg, of course!
France and Castile were long standing allies, before beginning clashing in the last years of the 15th century. Aragon and France were several times in conflict and sometimes allied, and It was their rivalries in italian affairs that brought the long-lasting conflict between Spain and France.
Exactly, the French-Spanish rivalry came from a Aragonese and Burgundian heritage. The wars between Spain and French begun in XV century and finished in 1814.