I hate to criticize the mistakes of a basically good pope, but now that Roberto de Mattei has done the unpleasant work for me, I can recommend his essay and endorse his conclusion.
The Third Republic was conducting a violent campaign of de-Christianization, particularly in the scholastic field. For Leo XIII, the responsibility of this anticlericalism lay with the monarchists who were fighting the Republic in the name of their Catholic faith. In this way they were provoking the hate of the republicans against Catholicism. In order to disarm the republicans, it was necessary to convince them that the Church was not adverse to the Republic, but only to secularism. And to convince them, he retained that there was no other way than to support the republican institutions.
In reality, the Third Republic was not an abstract republic, but the centralized Jacobin daughter of the French Revolution. Its program of secularization in France was not an accessory element, but the reason itself for the existence of the republican regime. The republicans were what they were because they were anti-Catholic. They hated the Church in the Monarchy, in the same way that the monarchists were anti-republican because they were Catholics who loved the Church in the Monarchy.
The encyclical Au milieu des solicitudes of 1891, through which Leo XIII launched the ralliement did not ask Catholics to become republicans, but the instructions from the Holy See to nuncios and bishops, coming from the Pontiff himself, interpreted his encyclical in this sense.
I’ve read in several books that Leo’s hope was that Catholics would working through republican forms vote the monarchy back in, but I’m not sure what this is based on.
As de Mattei shows, appeasing the Left had the same effect for the French Church that appeasing the Left always has.
Despite Leo XIII and his Secretary of State Mariano Rampolla’s endeavor, this policy of dialogue was a sensational failure and unable to obtain the objectives it proposed. The Anti-Christian behavior of the Third Republic increased in violence, until culminating in Loi concernant la Séparation des Eglises et de l’Etat on December 9th 1905, known as “the Combes law” which suppressed all financing and public recognition of the Church; it considered religion merely in the private dimension and not in the social one; it established that ecclesiastical goods be confiscated by the State, while buildings of worship were given over gratuitously to “associations cultuelles” elected by the faithful, without Church approval.
de Mattei then speculates that it was the rise of a more combative pope that prevented the most draconian aspects of the separation from being fully implemented. He concludes
The spirit of ralliement with the modern world has been around for more than a century, and the great temptation to which the Church is exposed to, is still [with us]. In this regard, a Pope of great doctrine such as Leo XIII made a grave error in pastoral strategy. The prophetic strength of St. Pius X is the opposite, in the intimate coherence of his pontificate between evangelical Truth and the life of the Church in the modern world, between theory and praxis, between doctrine and pastoral care, with no yielding to the lures of modernity.
It’s always a mistake to think the Church should stand above arguments over whether the civilization she founded should be destroyed or preserved, i.e. that she should avoid being Left or Right. The Church is a Rightist organization, obviously, because preserving the Christian principles of hierarchy and duty is the definition of the Right. The Left never, ever reciprocates gestures of conciliation. When it senses weakness, it strikes. It’s painful to admit it, but this really should have been obvious even in 1891.