In OTL, Spain stayed out of the war because they were still recovering from their own civil war. Could the Fascists to have come to power without causing as much damage? Maybe a more successful insurgency or even a democratic election?
If Spain did join the war, fighting for the Axis, how would that have changed things? I'm picturing a more difficult situation for England, and Americans becoming very distrustful of Spanish speakers. I also wonder if Spain would occupy part of France, freeing up German troops to fight elsewhere.
So the challenge is to put Spain in the Axis and discuss the consequences. The only rule is no ASB.
OK, here's one idea, but with a 1910 POD that, outside of Portugal, doesn't cause too much butterflies.
*The Portuguese First Republic is much more stable than in OTL. For that to work, let's say that cooler heads prevail and Portugal has a friendly separation of church and state, instead of the OTL hostile one. This, among other factors needed to get it right, prevents the rise of Salazar.
*Because the Portuguese First Republic (in TTL just the Portuguese Republic) is much more stable, its economy is able to expand very quickly. Which not only gives it a bit more clout, but in TTL it could ironically make Portugal even
more closer to the UK than OTL. For example, Portugal adopting the British 405-line TV standard, in this case, wouldn't be far-fetched. If the Commonwealth exists in TTL, it also wouldn't be far-fetched to have Portugal as a member of it - and thus participate in the Commonwealth Games.
*Due to Portugal's success, it might exacerbate things in Spain, thereby making the SCW much more bloodier than OTL if Franco wins. Franco soon makes irrendentist claims on Portugal. When WW2 comes around, due to that, Portugal joins the Allies very early on (due to the British connection) - as a result, Spain joins the Axis.
*This makes the Iberian Peninsula another front. With Allied help, Portugal curbstomps Spain and thus forcibly decolonize it - in some cases, by making those colonies independent or potentially taking them for themselves as war booty. Portuguese, in TTL, thus replaces Spanish as the fastest-growing world language - albeit in its European, not Brazilian, form.
*The Spanish Republican government-in-exile is eventually invited back to form a government; from 1945 onwards, Spain is now a republic whose right wing is largely discredited with bringing about ruin for no gain.