I agree Nishant, to me chapter 19 of the Apocalypse appears to blatantly refer to a Great Monarch, and the "angel" of chapter 18 to the Angelic Pastor. However, I don't know what the Church says about these passages -- if anything, it's clear that the Apocalypse does not have an official interpretation for many parts. So while to me it seems very clear, other things have seemed clear to me before where I was wrong... Just trying to stay open-minded.
I now believe in the Great Monarch again, but I have studied it much more thoroughly. What Belloc says about how many people have prophesied the Monarch is how I thought before, but it is not true -- there are actually very few, many quotes are doctored or apocryphal or taken out of context. However, I have verified at least two important prophecies, those of St. Francis de Paola and Holzhauser. They are both extremely detailed and both very real, the sources are authenticated. Cornelius a Lapide even wrote a commentary on the letters of St. Francis de Paola. Holzhauser made some false prophecies -- he said Antichrist would be born in 1855 -- but also some true ones.
So now I have a different slant on the Great Monarch theme. I think that God is keeping it more secret, just as he did the arrival of Joan of Arc, who certainly did not have many saints predicting her advent but only some vague village-level prophecies. The reasons are obvious, if the way the Monarch was going to arise was too well-known, the devil could use his agents to stop him.
Which is happening anyway, to some degree. I now see that others are heaping disinformation on top of the REAL Monarch prophecies, perhaps even to make the Monarch appear as the Antichrist, which I fell for. For instance, two lines were added by someone to the letters of St. Francis de Paola, saying that the Monarch would inaugurate a peace that would last until the end of time. That is fake.
Much discernment and prudence is required with this topic, that is for sure. The big names who invoke the Monarch, like Yves Dupont and Marquis de la Franquerie, are in my opinion not the best sources.