AHC: Franco-Spanish front in either World War

With a POD no earlier than 1900, make it so that the Pyrenees are a front in either the First or Second World War. In other words, Spain doesn't maintain neutrality in the wars.

Any plausible ways to do it?
 
With a POD no earlier than 1900, make it so that the Pyrenees are a front in either the First or Second World War. In other words, Spain doesn't maintain neutrality in the wars.

Any plausible ways to do it?

Franco-Prussia War breaks out over the Spanish Royal Crisis of 1868. France loses, as per OTL, and Spain is Hohenzollern and German-allied.

Holding the rest of history until 1914 constant, when WWI breaks out Spain joins the war.
 
Would France be as militantly gung-ho if it’s guaranteed that she’ll be trapped in a vice in the case if a second war with Germany?
 
Would France be as militantly gung-ho if it’s guaranteed that she’ll be trapped in a vice in the case if a second war with Germany?

I mean, wasn't Bismarck's diplomacy partially based on preventing a two-front war by keeping France diplomatically isolated and Russia pacified?
 
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