Well, I'm from Spain, and I haven't read the novels yet, but it seems my country isn't represented in the series. This got me thinking, "What would the history of Spain be like in Youjo Senki?"
To begin with, in Youjo Senki, it would probably be called Hispania or something similar instead of Spain. Let's assume that history has remained almost unchanged up until the point where the series begins.
The republic would still be in power, but since Communism in Youjo Senki hasn't spread ideologically as much as it did in the USSR, the republic's policies would be much less communist and probably more socialist or conservative.
So, at the beginning of the series, Hispania has two options: choose a side or remain neutral. It seems they choose the latter.
But here comes the big change: the coup d'état led by Francisco Franco would fail or simply never happen. Without the support of the Axis powers (Nazi Germany and Fascist Italy), the Nationalist side could not have won the war. The more moderate practices of the Republic would also have resulted in less internal support for the regime change.
With the coup failing, there would be no dictatorship, and Spain would not be isolated from the rest of Europe. It would industrialize faster, following the European trend, now integrated, without so much internal tension, with institutional confidence, and without successors of Franco exploiting state mechanisms for personal enrichment.
Spain's economy would not necessarily be larger than it was, but it would be much more robust, and it would be a consolidated geopolitical force.
It is curious how simply prolonging the First World War could have so drastically altered the fate of an entire country.
This would happen even if Hispania chose a side during the events of the series. Regardless of which side, the empire wouldn't allow a coup that would cause them to lose an ally with such a strategically advantageous geographical position.
And the same applies to the allied forces. None of the players in the grand game of Youjo Senki would support a coup in one of their allies, as there isn't the same level of ideological pressure as there was in the real world.
Honestly, if the author ever decides to do a spin-off, I think the Iberian Peninsula has a lot of potential.
If I remember well, I think Spain(and Portugal) is called Ispagna or something like that. The only think they said is that the nation is neutral because some internal political problem, maybe a civil war but nothing is confirmed(if I'm not wrong). But there isn't any more information. My teory is that after a kingdom with a dictatorship(Like Primo Rivera in Spain) a republic might be stablished. But a civil war may start, the sides would be like in the Spanish Civil War(if the communist do not have strong power in the world of Youjo Senki, I would be nice to see the anarchists took their place), that can explain why Ispagna does not go to war.
I think the other countries won't take part in the civil war but they would have a know side to support, leading to a new kingdom or a dictatorship(I think there isn't a equivalent of fascism in Youjo Senki, but I am not sure).
I like the idea of a spin-off, maybe Tanya could be sent to Ispagna as a way to the Empire to secure that the side they want wins the civil war. However the damage to the infrastructure wouldn't allow Ispagna to join to war in the side of the Empire, like in real life Franco didn't join the Axis power and remain neutral only sending some resources and men.
The unified states and Latin America, seeing that the Republic of Hispania joined the empire.
Unified states: Oh no, do we have to get involved too?
LATAM: Oh no, we have to send volunteers.
True Latam: They will finally be able to return the gold (Spain never took it, it was the English).