Now, as we all know, the True Faith was spread over Europe, and the Eastern region before the Muslims invaded it, almost entirely by peaceful evanglization. Wars with the aim to Christianize the population were few, and even so, the majority of said wars were defensive wars that later, and justly so, turned into offensive wars, such as Charlemagne's war against the Saxons to Christianize them (If you don't know, Charlemagne's casus belli was that the Saxons started the war by sacking a church within his territory.)

Now, as we also all know, contrary to all the spin secularists try to put on it, secularism was spread mostly through war and bƖσσdshɛd.  The first thing the vile French Revolutionists did after their bloody conquest of France, which resulted in the beheading of many French aristocrats, regardless of whether they were actually guilty of corruption or not, was invade other territories that would not submit to their dogmas of secularism and so-called 'enlightenment', to try and force these things on other peoples too intelligent to adhere to them without being smashed in the head with the butt of a musket rifle.  One of these territories was the Papal States, which ended up with the Papal States losing some of its territory.  Some more conquests, such as those of Napoleon, which successfully, though violently, spread secularism to a lot of places, and nearly destroyed the Papal States, and even more so by Victor Emmanuel, and the Papal States were lost for good, and were driven back to what is now the small, nearly powerless, city-state of Vatican City.

Now, here's my question. The Papal States were taken through absolutely nothing other than violence, force, and conspiracy, so, in the future, if a strong Catholic nation is formed by the will of God, why exactly should we stand for it?  Would a Reconquest of those territories be justified under the Just War theory, or has it been too long, that now, we should accept those territories now belong to the secularists?

Who do you propose would do the reconquering?

Any attempt to "reconquer" the Papal States would necessarily be a war of aggression and not a just war.  It has been 144 years since Rome fell to the Italian nationalists, and 85 since Pius XI singed the Lateran Treaty and recognized the legitimacy of the Italian state, including its sovereignty over what once was the Papal States.

Unless you believe that Pius XI was a modernist anti-pope, acknowledge his authority to do what he did, and move on.

Totally agree. And Pius XII never questioned the Lateran Treaty. In actual fact, he ensured that the Treaty would be protected under the Italian postwar Constitution.