From: Republic of Cascadia
Speaking from experience, I have to disagree with the assertion that France would necessarily have enough troops to actually execute the conquest of Spain.
In one of my AI games, the AI managed to botch Schlieffen, leaving the corps in Brussels quite healthy (I left, though) and just barely managing to take Reims. However, the pressure intensified, and by mid-1915 most of my troops were half-health, there were gaps in the Champagne sector, and I almost lost Verdun and Nancy for good. I also had the Italians, who reached Lyon, on my back and sent, yes, five corps to deal with them. I was saved by Russia, which managed to take Oppeln and secure every town in Galicia except for Tarnow, and Romania.
In a human game, the pressure would be even more intense because there's no way Schlieffen would be botched like that and, as a result, everything up to Amiens would be indefensible and France would be even more mauled. Italy's neutrality would allow you to send troops elsewhere, but you would have less to send. Not the five as planned.
That said, Spain has three major problems: its army is terrible, its troops are dispersed, and strikes. Indeed, Spain's army being terrible is historical, but it knew about it; during the war it increased its army to 180,000, an increase of 40,000, or two in-game half-strength corps. And meanwhile, if Spain is to join the war, it would also move its armies to the borders, not the weird in-game distribution with a third of the army in Madrid. The equivalent is moving three corps from Lorraine to Orleans or from Galicia to Hungary, which is ludicrous at best. And, apparently, there is no one leading all of these forces or planning the war. But it gets worse, as every time Spain is attacked the UGT and CGT cast their vote for French domination by devastating supply along Spain's most defensible feature, the Pyrenees.
So yeah, Spain needs buffing, but it would be amazing to see France pull a proper defense during a historical-esque playthrough. The idea of Spain being a neutral major is good, but it would also need to go to war in a semi-ready state and a certain amount of tech just to survive. And if Spain favors one side initially and goes to the other, it might result in one player spending MPPs for their opponent.
These are all good points. This 'Question of Spain' has turned into a really interesting discussion.
One thing that came to my mind was that there is a precedent of Spain being attacked from a "to be' great power, and that is the USA in 1898. The Spanish-American War was according to some views, a naked aggression. What ever view is held in this matter, the result was that major strategic and monetary gains were made by the USA.
Now that event is recent history in the timeline of this war, so I'm beginning to think an Entente attack on Spain isn't such a far fetched idea after all...which leads to a possible 'buffering' or 'beefing up' of this country's OOB and or other factors to mitigate a fast rollover of this state.
Leave it to the players of this great game to push the envelope to see what gives.