It is important to note the differences between the Kingdom of Poland and that of Spain — that is to say — the differences between the situations of Henri III and Felipe V of Spain.
Poland was an elective monarchy. Henri III was forced to sign the Pacta Conventa, in which Henri acknowledged the system of free election and rejected any attempt to form a hereditary monarchy. He also issued the Henrician Articles, which effectively served as Poland’s constitution until 1791.
This meant that it would have been unlikely that any children of Henri III would have become kings of Poland.
Spain, on the other hand, was a hereditary monarchy, and it was formed by a union of the crowns between Ferdinand II of Aragon and Isabella I of Castile. In fact, it was Felipe V of Spain who officially abolished the individual crowns of Aragon and Castile to form the Kingdom of Spain.
With Spain being a hereditary monarchy, it was clear that the sons of Felipe V would succeed him to the crown of Spain.
So, we have one kingdom which is elective, and the other hereditary. One where potential children will not succeed to establish a dynasty, and where where children will succeed and establish a dynasty.
This is a fundamental difference, and so, we see a fundamental difference between the situations of Henri III and Felipe V.
Both Henri III and Felipe V were given letters patent to maintain their succession rights.
Under Henri’s situation, it was clear that any sons born to Henri III would most likely return to France. Of course, he never had children, and it was Henri himself who returned to France upon the death of his brother, Charles IX.
But Felipe’s case was different. His children would (and did) succeed to the Spanish throne, meaning they would remain in Spain. Thus, it was wholly inappropriate for Louis XIV to have given letters patent to Felipe V to maintain his succession rights and those of his children.
It is inappropriate because there would be a prince who was a Spaniard, a prince who could be nothing more than a Paper Frenchman, all because some document issued generations earlier declared him and his ancestors from Felipe downward to be “French.” But in truth, they are French only on paper.
Further, unless said prince were to give up the throne of Spain, then there would be a union of the French and Spanish crown. This fear is what caused Britain to declare war on France and Spain to prevent such a union.
What would such a union mean? Spain was a powerful country with an empire. Would this not make France a de facto vassal of Spain?
The result of such a union is exactly what was opposed by Claude de Seyssel:
And the first specialty that I find good there is that the kingdom goes by male succession, without being able to fall into the hands of a woman, according to the law that the French call “salic”, which is a very good thing. Because, falling in a feminine line, it comes into the hands and power [it can come into power] of a man of strange nation, which is pernicious and dangerous thing: yet that which comes from such a strange nation [the one who comes from strange nation]is other food and condition and has other mores, other language and other way of living than those of the country where it comes to dominate.
The words of Claude de Seyssel are proven right by the words of Carlos, Duke of Madrid (de jure Carlos VII of Spain) in December of 1887:
Before now I said that I would never leave Spain, and today I repeat it. I am bound to their destinies by the torrents of generous blood that I have seen spilling in my defense. I swear it one more time: I will never abandon it.
What we would have is a Spanish prince with Spanish ways and a Spanish custom becoming King of France. It is a scary thing, for the French fought the Hundred Years’ War to keep the Plantagenets off the French throne.
But even in the Plantagenets’ case, they were far more French than actually English. They even ruled half of France at one point. Regardless, they were rejected because they were foreign. And since then all foreigners were rejected.
But in examining the Spanish Bourbons, what do we see? Spaniards! All Spaniards! For goodness’ sake, Don Luis Alfonso Gonzalo Víctor Manuel Marco de Borbón y Martínez-Bordiú is the great-grandson of the great El Caudillo himself, the man who saved Spain from communism!
Thus, it was very improper for the letters patent to be issued to Felipe V, and in doing so, Louis XIV only made it harder on the military of France.
Those letters, of course, were revoked. This means that in custom, law, and paper the Spanish Bourbons are foreign, having settled in and being born in Spain.
In 1883, the last of the elder Bourbons in France died. The Count of Montizon, despite being the senior agnate of the Bourbons, was a foreigner and a Spaniard, his family having lived in Spain for more than a century and himself being descended from three generations of foreign-born Spanish princes.
Thus, under the Fundamental Laws, which exclude foreigners from the French throne, the House of Bourbon-Orleans became the Royal House of France.