Spain’s economy was non existent due to the Spanish Civil War, they did not have the capability to go to war with anyone nor did they have the capability to defend themselves. In the end this would just make Germany’s situation worst as they would have to keep more soldiers in the West which would give the Soviets a large advantage. Notice how the Germans had a hard time maintaining enough soldiers in the west to defend coastal areas in the event of a amphibious invasion? You just made this problem worst. What would happen after the war? Spain is likely to be occupied by the allies either that or it switches sides after another civil war due to the fact that Franco just went batshit insane and had his country fight another war.