Make an Iran-esque revolution occur in Italy.
Very though.
My first thought was to call ASBs on this.
However, here's my take:
In 1940, Ciano talks Mussolini out of the idea of declaring war. Italy focuses on strengthening the grip on East Africa and Albania and looks around anxiously for cheap opportunities at landgrap, but stays out of the mess, although quite German-friendly.
Hitler has little to complain about it.
However, pointing out the dangerous political situation, Mussolini decides he has the excuse to enforce measures that ramp up control; censorship is tightened, opposition persecuted more thoroughly, the scope of anti-Jewish laws widened, and so. Among these measures, control of education by the State is reinforced, with the request that Catholic school programs are controlled by the government. This is accepted, but it's a stretch under the Concordat and many clerics are pissed off.
At the same time, bilateral relationship between Germany and the Vatican worsen. Feeling a bit more secure as Italy is not at war, Pius XII resolves to emit a more open condemnation of Nazism than he did OTL, though he's unlikely to go as far as saying that any Catholic who cooperates with Nazis is automatically excommunicated. This causes, as he had foreseen, a backlash against Catholics in German-occupied Europe and some more fault lines emerge; the Church is less pliable to condone crimes by the pro-Nazi regimes in Croatia and Slovakia, which in turn do far less of a pretend of Catholicism. All in all, this means that a solidly anti-fascist, persecution-hardened component of the Church will be there after the war.
The war goes broadly similarly to OTL, minus the East African, Libyan and Italian theatres; France lasts marginally longer in 1940, and is more riven by contrasts; Germany does marginally better in Russia thanks to the offensive starting a little earlier and more resources not spent in Africa and Greece. Jugoslavia is invaded more or less on schedule for similar reasons, and while remaining neutral on the whole, Italy might get a share in Dalmatia. Italian volounteers fight in Russia ITTL as well. In the big picture, however, this is not enough to change things in the end. Russia is even worse off than OTL, but the Western Allies don't fare so well in *Normandy. Berlin falls just in time to avoid a nuclear blast in summer 1945.
The Pacific War goes broadly like OTL.
So it's 1945, Nazi Germany has been just horribly wiped out and Mussolini is trying to sit out the mess hoping no one notices. He has used this time to improve the military, at least on paper, trying to puppetize Greece as discreetly as possible (not working that well) and rooting out of Italy any political, social or cultural force that isn't Fascism. He's been fairly successful, and with less brutality than, say, his colleague Franco. The only force he couldn't simply suppress is the Catholic Church, but the mutual attitude has become incrasingly confrontational.
As the rest of Europe is pretty much a wreck in 1945 and Italian economy fares relatively not so bad, industrial exports boom, allowing Italian industrialists and the Fascist Party leaderchip to get filthy rich.
Il Duce decides that the US are his new best friend and in the new Cold War context is the Western proxy for the ongoing vicious civil war going on in former Jugoslavia, where he hopes to keep his ill-gained grabs and if possible expand them, or at least instate subservient regimes in small successor states. This devolves into an unholy mess where nobody is sure what the hell is going on and why, except that the US are supporting whoever is not Communist, including support for every brand of Croat, Serb, Slovenian and Macedonian nationalists at each other's throat.
The unclear situation makes Stalin unwilling to commit troops directly, but the chaos is such that many expect WIII to begin in Sarajevo by 1948.
This is avoided and Jugoslavia is finally divided into Communist Macedonia and Serbia, a quite oversized monarchical Montenegro, a rightist, authoritarian, but not exactly fascist Croatia that includes most of Bosnia (other parts go to Serbia, including Sarajevo, and Montenegro) and neutralized Slovenian republic that looks worriedly at Italy (who got a slice of Kosovo) and so tries to make best friends with the democracies. Bosniak Muslims got the short stick and many emigrate to Turkey. Those who stay are in for quite difficult times, varyingly more so in the Serbian or Croatian territories.
This dirty war has been pretty unpopular in Italy, with the Church as the only organization able to voice some degree of dissent; this makes Mussolini even more confrontational, arresting prominent former Popular politicians and any cleric he does not like.
In prisons, Catholic anti-fascism elaborates (possibly both in Italy and Croatia). Building on the Popular roots and the thought of Luigi Sturzo and Primo Mazzolari, a corpus of reflections on the relationship between Catholicism, Fascism and society develops. Some priests are exiled rather than imprisoned and end up in Latin America, where a movement analogous to OTL Liberation Theology will appear. Some books are circulating clandestinely. Finally in the sixties, a young, smart priest called Lorenzo Milani (OTL character; although his life here is very different) after having spent years in confinement and prison in Italy and then as a missionary in the poorest favelas of, say, Sao Paulo publishes "Church, State, Justice". He says that the States is a powerful instrument of modernity, that could ensure betterment in the life of many people; but it is also a moral agent, inherently tyrannical and unjust. Only the Church possesses an approximation to true justice; it is therefore its duty to overthrow tyrannical states and assume control of the government machinery directly.
The book is incredibly controversial within the Church and is obviously banned in Italy. Milani however is a talented polemist and orator and writes very convincigly.
He's a passionate believer in his view and has a wide culture.
Soon his ideas gain, if not acceptance, audience. Catholics have an anti-Fascist ideal rallying point.
The regime cracks down more heavily as a consequences, bordering on persecution as far as it can without risking open revolt; but the aging Mussolini and his increasingly subservient cronies have lost touch with reality. They start interfering with things like appointment of bishops and festivities.
Things worsen throughout the late sixties and early seventies, with the Fascist regime increasingly unpopular and decrepit, Italy more and more isolated from the world, the economy strained bu unwinnable conflicts in Africa and what is gained from Libyan oil enriching only a few leaders.
Milani's militant Catholicism is the only serious rallying point for the masses.
Mussolini is dead and his successor (let's say he's Farinacci) acts even more ham-handedly in suppressing Catholic dissent. At some point, he does something highly provocative and stupid that causes public protest. Police fires on the rallyings. The government thinks some grapeshot will give those cowardly priests and their timid flock the therapy they need. On the contrary, protest increases.
The Pope declares the dead are "Martyrs".
Things start really go downhill as almost simultaneously, Portugal and Spain are undergoing a similar process, although not led by militant Catholics.
Rallyings become urban fightings. Policemen start to refuse to gun-machine their own people. Soldiers start to mutiny.
The Catholic Revolution has begun...