Question for scholars of Spanish/Portuguese history: Why is Galicia part of Spain and not Portugal?

Your knowledge of Iberian History is appalling, guiri. You missed every single thing.

About your question, is rather complex. The common coastal geography and language has little to do, since political, military and religious factors have far more importance. Actually the river Minho separates northern Portugal and Galicia.

Basically speaking, back in the Middle Ages from the 9th century on, there were several royal and local dynasties that fought for supremacy in the north-western Peninsula. Sometimes the dynasties were partitioned, weakening the regional powers, sometimes they were reunited, increasing the power of one particular royal family.

"Portugal" itself was a typical medieval bordering county, a march/mark, under the dominance of Galicia, that itself was sometimes independent, sometimes under control of other kingdom. So in the late 11th century, the Portuguese county and Galicia were separated into different political entities, with some defined territories. In the next century, both areas among the other political sides, either Christian or not, tried a reorganization of their regional status.

We can say there were two main factors that separated Portugal and Galicia local powers for good:

one, of political nature, when two factions fought for each other for domination; the Portuguese party, so to speak, won against the Galician party, both in military ways and in royal sucession, that established a new dinasty for Portugal. Despite this, later Portugal did tried to conquer its influence in Galicia too, but without success. Galicia tried too to take some control of the Portuguese court later on as well. But as the centuries gone by, both areas became uninterested of such intentions.

other, of religious dominance, mainly between the Sees of Braga and Santiago de Compostela, regarding their control over their territories. Their local powers were quite significant, so it actually took more time than the first factor until both See areas corresponded to their political counterparts areas.

The separation of Galicia and Portugal has little to do with culture and language, it was more a local redefinition of the balance of power. Had the Portuguese party lost their military supremacy in the 1120 s and the next decade or had a weak royal family, the area hardly could surpass their march status. The thing is, the first Portuguese king was a military leader, and that it was in the end what allowed him for political status; on the other hand, Portuguese ascension from a march into a kingdom created another local power and Galicia itself lost their political status, more dependent on other kingdoms until being subdued to "Spanish" dominance.

According to experts in language, Portuguese is a dialect of Gallego.

Spaniards say that Portugal just was not nice enough to colonise. And let's face it Spain could have taken it over whenever it wanted.

Obviously, there are historical reasons why it never happened - treaties such as the treaty of Tordesillas etc. etc.

Why is new mexico part of the USA ?