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The Dutch are probably going to end up on the wrong end of an 1890s version of Market-Garden once the French feel they've run out of better options.
I'm amazed the whole Belgian situation hasn't blown up spectacularly yet; what precedent is there for a nominally neutral country to allow armies from one belligerent to cross its territory on foot to attack another?
Honestly I can think of one off the top of my head, but it's very anachronistic--Cambodia under Prince Sihanouk during the Vietnam War. But of course Nixon's eventual response to that was to sponsor a coup that threw him out and brought the Cambodian government in on the US side--only to be overthrown by its own homegrown insurgency of course. Sihanouk was in a position to disavow any connivance with the Viet Minh. How can the current Belgian government do the same?
I presume the main reason North Germany and the UK have not declared war on Belgium is that they feel they have their hands rather full at the moment and don't need yet another helping of open warfare on their tables--but the current Belgian government is on very very thin ice with them I'm sure and the moment the two northern powers feel they can afford to they're going to be making some stringent ultimata to Brussels. Or look very very very favorably on a change in government there. If the BOGs win through to a peace that is anything more than a draw, the current regime in Belgium is going to find itself with very little diplomatic capital.
And I got the sense when last we hearing about it, that the government's choice to turn a blind eye to an army marching through was very controversial and quite unpopular with a lot of people.
So the French might decide it would be brilliant to sweep through the Netherlands and strike yet another vulnerable flank of Germany--which would bring the Dutch in on the BOG side of course, unless the monarchy decides to order a surrender to French occupation a la Vichy and even then there is the question of whether Dutch holdings overseas would obey or not.
But first they have to move an overwhelming army to strike with surprise--the current Belgian government might assist them with that (and surely have to figure they are openly on the FAR side after that)--but I'm not sure the government could rely on all the Belgian people standing aside and letting that happen without protest, perhaps even fighting it openly. If civil war breaks out in Belgium over the question of French troops crossing it, that's a warning to the Dutch; they can probably hold for a little while, long enough for North German and British reinforcements to come in. And perhaps under the circumstances, be rewarded for turning their poor country into a battlefield with the promise that Belgium is going down and Flanders will wind up a Dutch territory. (That would create an awkward post-war problem though; the Francophone part of Belgium would surely then gravitate toward being annexed to France, a peculiar "penalty" for France to suffer for losing!

Trying to maintain a rump Walloon mini-state would be a thankless task; they'd be trying to Anschluss to France and in lieu of that, acting as French proxies as long as the separation was enforced.)
So--the current regime in Brussels has pushed it about as, um, FAR, as they can dare short of openly joining the FAR alliance. If the Belgian people overwhelmingly wanted that they'd probably have done so. I don't know if more "Papal Legionaries" are going to cross into Germany via Belgium--but I don't think the French can
quietly move a sufficient army to rapidly subdue the Netherlands across Belgium without an alarm going off.
They could of course make a pretense of conquering Belgium with that same overwhelming force, which the current regime might surrender to with unseemly haste and yet claim plausible deniability. But that process, however well-orchestrated, will still take some time and its start is the signal for the Dutch to mobilize. If they haven't already, citing a need for armed defense, pointing precisely at the precedent of Belgium allowing the Papal Legions to cross into Germany.
And if nothing else, the Spanish are likely to join in once it becomes obvious that FAR is going to lose the war, in the hopes of grabbing some spoils and a seat at the peace conference.
I'm not so sure that it would even be in Spain's rational, cold-blooded interest to do that. Just what would they be looking to grab so opportunistically? Territory in North Africa, perhaps? Don't they have a handful of that already? The British are not going to offer any deals regarding Gibraltar. There are some territories which have oscillated back and forth between Spain and France.
But I'm not sure any of that is worth infuriating France, even when France is defeated for the moment. Spain is in the same position Franco's Spain was in in WWII, it looks to me--she really can't afford to piss off
either bloc. Unlike Falangist Spain, which brutally suppressed dissent, there are probably strong voices on both sides--but anyone advocating open alliance with the BOGs is probably in the minority.
I'm admittedly projecting backward from the OTL Civil War that was 40 years later and in our timeline, not this one--but if there is anyone in Spain who is not going to line up with the clericalist, reactionary side that wanted to commit to FAR years ago, they strike me as unlikely to be comfortable with the staid powers that be of the BOGs either. They'd more likely be countryside, urban, and separatist radicals who are much more
against Madrid than they are
for the interests the northern (and Protestant) allies of BOG are. They might be anti-clerical--to Britons and Germans, they would be perhaps to a fault. These explosive social elements won't be conducting a cold-blooded policy of coming late to the war to win spoils for the Madrid regime; they'd be revolutionaries, half committed to destroying Spain as a unified nation, the other half committed to so transforming it their rightist foes would deem it tantamount to the destruction of Spain wholesale.
I suspect that maybe in this time period so much earlier, radicalism might not have evolved nearly so far yet and there might be a substantial liberal bloc. But their interest would be to stand pat, profit as neutral middlemen, keep their tenuous grip on their distant colonies secure by not offending the British--and as much as their more reactionary counterparts also committed to Spain as a nation, keep whatever degree of radicalism as might already have fermented in check.
These hypothetical liberals might be currently holding a tense and uneasy balance between these radicals and the ultramontaine clericist Right, that boils over in anger over both the treatment of the Pope (currently residing among them and spewing venom and anathemas against the unholy BOGs) and Spain's long-held grievances against British ascendency. Never mind the Peninsular War, nowadays the FAR alliance, conveniently ignoring the ancient schism with the Eastern Orthodox (who also after all call themselves
Catholic, and are currently fighting, as the rightist would have it, for Right Reason) is, a Holy Alliance of true religion and conservative order against depraved modernity and apostasy. We've all noted before that religious allegiance is more of a factor in this timeline at this point than OTL. In Spain, I think this means that it would be impossible for Spain to war against France without civil war. Not only are the two north European mainstays of BOG Protestant, and Italy the victim of wayward folly of her liberals damned and doubtless by now formally excommunicated by the Pope they betrayed, the third core member of BOG is of course the Islamic Ottoman Empire.
The crusader faction has not got its way, but I do think they would veto any thought of actually attacking the FARs and consider it a shame and scandal they aren't aiding them.
The radical faction is perhaps not yet sufficiently developed to fight and win a civil war, and if they were the BOG powers would be reluctant to invoke them. If the clerical-crusader faction did get their way and plunge Spain into war against BOG, damn the rational considerations, the British just possibly might try to set that bomb off, in the same spirit the German General Staff OTL sent Lenin to Russia. But they might be regretting it for decades to come.
Aside from these hot-blooded factors, Spain just can't afford long-term animosity with either side. They need British tolerance to keep their empire and indeed to import sufficient goods--and for that matter the French would much rather they stayed neutral so they could go on channelling trade goods to them as well. And they can't afford, even if domestic politics would not forbid it, to earn the long-term hatred of France either; France is right there on their border and will be for as long as either nation lasts.
Either Spain goes nuts and goes FAR, or they stay neutral to the end, is my take on it. They can't go BOG and stay in one piece.