Taking into account that Imperial Russia was no more and that Britain had already starting clashing with France diplomatically, then Belgium should have been courted not only in the economic sphere as I mentioned above but also in the diplomatic one.
I am curious how you think France could have wooed Belgium better diplomatically?
About all I can think of is France somehow outlasting Germany so that the Rhineland occupation is seen as a narrow French/Belgian/Italian win. Elsewise, it seems to me that France is just unable to offer Belgium enough (since Belgium wants British commitment and the French can't really control the British).
Do you have in mind any particular POD for it? Were they any reform efforts to tackle the instability?
I know you aren't asking me here, but since I have some thoughts as to potential PoDs, I'll butt in anyways.
There were people with ideas on the right lines, and most of them are the sort of people who might end up joining a French Christian Democratic party, so I have always been rather drawn to that as a PoD. The best PoD I've found for that is pre-WW1 though, and since I don't have a great deal of interest in slogging through an alternate WW1 to write a timeline about an interwar France with a Christian Democratic party, it's one of those ideas I've shelved until I can find a post WW1 PoD.
As far as specific policies that could have stabilized French politics... Well, France didn't actually have unstable politics - yes, governments were very short-lived, but if you look at who the "big beasts" in the different governments were, you keep seeing the same fellows doing the same jobs across several governments before moving to another post of high office or taking a break for a couple years before returning to high office. Indeed, in some ways the system was too cozy and stable. However, what is true is that
fear of instability was a real problem. Blum kept out of the Spanish Civil War due to fears that intervening would start a
French Civil War. The press was constantly filled with extremist doom-mongering about the political groups the paper in question opposed (one of the reasons why some factions in France almost welcomed German occupation is because at least the Germans weren't the baby-eating mass murderers of the "left"). The military was intentionally undermined as an effective fighting force in certain respects due to the fear that this or that part of it had sympathies to this or that political party.
Also, part of the reason for apparent instability is that national politics was not as professional as it was in Anglo-Saxon countries. A member of the parliament of the 3rd Republic would usually have another job that consumed the majority of their time, such as being town mayor of somewhere or a businessman or (as during WW1) a soldier. Parties were much less formal than they were in the US or UK, making pro-active politics very hard. Lastly, the majority of politicians were lawyers, meaning they had little in the way of experience of other fields besides politics and law and thus were full of terrible ideas about things like industrial policy and finance. So the chamber of deputies and French senate did not have as much ability to exert control as the political systems English speakers are more familiar with.
That said, there are things that could push France down a better path, though these things are major changes to the status quo and thus it's hard to see them as being PoDs themselves.
First, is France retaining the income tax after WW1. In OTL this led to serious financial problems that led to a traumatic surge of inflation in the early 20s, that surge of inflation would then lead to the brutally tight policies during the Great Depression. At the very least, an income tax would lead to more reconstruction after WW1, more government economic activity and a more evenly-spread tax burden. At the most, it might reduce the level of the early 20s inflation surge and lead to a lesser reaction during the Great Depression.
Second, women getting the vote earlier. Women at this point in French history tilted towards being more socially conservative, more Catholic, but economically interventionist and uninterested in anti-republican reactionary politics. That is, women getting the vote in France in the interwar period would have meant a strong shift to the right and would most likely have meant a few center-right parties getting relatively high portions of the vote - and thus the political strength to push change and reform. Fear of exactly that is why French women didn't get the vote until 1944.
As for a POD, I think the Clemenceau government (post WW1) is the only one with the prestige and stability to tackle the problem as it was the war winning coalition government (Union Sacrée). OTL, they changed the electoral system for the 1919 elections (the previous one returned with some modifications in 1927). Clemenceau might get away with it as the reforms won't apply to him as he wanted to become "Président de la République" (and he was old).
I can't see it. Not only was Clemenceau the man most associated with an unpopular Versailles peace treaty, he had also imprisoned many political figures who shared his own political leanings in the last years of WW1. This had been done for fear that dissent at home might snatch defeat from the jaws of victory, but those Clemenceau imprisoned would not forgive him. He'd burnt his bridges and had no real prestige or political capital left. Maybe it would have been different if he'd been able to trick his rival Poincaré into being the top French representative in the peace conferences, but Clemenceau was from the most conservative part of the French political spectrum at this time. He might have claimed to be a radical, but it's not like radical republican positions on Boulanger and the Dreyfus Affair were relevant in 1919, while his party opposing the income tax and votes for women certainly was.
In OTL, France did not properly fortify their side of the Belgian border (which would have really ended the chances of the Franco-Belgian alliance being rekindled) and when WW2 started for them, they drove an important part of their forces deep into Belgium to try and protect the country despite lacking the coordination or military infrastructure to make defending eastern Belgium practical.
Actually accepting that France could not defend Belgium if Belgium wasn't an ally would have been far better than OTL.
fasquardon