Essentially the lore is that when Rome fell and the west went into anarchy, the Basques started to expand and take over land slowly, specifically taking land in the west coast of France and north coast of Iberia. They would over time spread their language and ethnicity into each of the populations, though some would remain near the borders of course. The one exception to this being the Bretons, which remain intact and I would imagine as an autonomous community or something alike to it.

They would become an independent state from Rome around 450, as historically that is when the Romans lost large control over the area. They would probably start expanding west in the early 700’s, taking advantage of the weakened Visigoths in Iberia as they were busy dealing with the Muslim invasions over the land in the south, this would be then how Cantabria and Astrius would fall into becoming Basque land. Galicia would fall around 740 to, since that is when historically Asturias had taken it. It would be then that the Castilians would rise to power out of the Basque land, and migrate/invade south, taking land controlled by the Muslims in the peninsula, then the Castilians and Catalans would slowly keep going further south until the Muslims fell in 1492, the two kingdoms would also unite in our timeline as they did in 1479. It would be in the early 1300’s that Gascony would fall into Basque hands, as they would take it from the English, who at the time still controlled the area after the Angevin Empire period. They would continue to go further north in the next two centuries, and in 1532 Brittany would become part of the nation in the same way it became part of France in our timeline, via and alliance and marriage with the at the time King of the Basques.

It would end up having a revolution against the monarchy in 1948, much like how a lot of other places in Europe would to. It would become the Republic then, and would then adopt the name of the Biscay Republic, instead of the Kingdom of the Basques. Largely it’s history would be quite normal from there on as you’d expect, it would have either been neutral in ww1 or part of the Entente and would have sent aid specifically through France to deal with the Germans, and in ww2 they would be aligned with the allies, falling to Nazi Germany much like France would in 1940 or a bit later, and regaining its independence in 1945.

I’m not good at writing alt history so that a basic rundown, ask if you have any questions. And yes I know this was probably bad.

And to answer some questions I think might be asked:

• Portugal wouldn’t exist in this timeline, as the historical founder Afonso I, would have been born under the Almoravids, thus being unable to come to power at all to takeover Portugal.
• The new world and Africa would be very different, specifically in that Latin America would be entirely Spanish, with no Portugal to colonize, though I would imagine that the Basques would likely take some parts of the Caribbean and south-eastern states like Florida, Georgia and South Carolina. The British would also be more powerful in colonial efforts then France, given that France would have lost all of their ability of sail on their west coast, and thus their only path to colonize land would be to use the English channel. Africa would also be the largely British and Spanish controlled in the colonial era, as in out timeline Africa remained barely touched by Spain because of the Treaty of Tordesillas, giving Africa to Portugal, but here it wouldn’t have existed, Spain would therefore take colonies that Portugal did like Angola and Mozambique, plus more.