This refers to something a bit grander than a mere “monarch” as head of state over a single nation, to be fair. It conjures up the image of an empire which - at least nominally - exerts some kind of suzerainty over international Christendom as a whole.
I must admit that, while I was aware of the prominence of this idea in Christian thought throughout the ages - e.g. in the Constanian era under the late Roman Empire, the medieval Carolingians and in the works of the Italian Florentine poet Dante - I honestly never knew nor expected it to feature in any discernible way in the liturgy.
This has come as a real surprise to me. I actually re-read those passages in the missal numerous times, partially disbelieving what I had actually read. I would not have thought that the Church paid this pious, largely theoretical and quite utopian vision of a peaceable, imperial
Res publica christiana - which never really transpired in practice, at least in terms of its global ambitions - such honorific mention during actual Masses.
I had been under the impression that it was just a medieval cultural zeitgeist - folklore, imperial braggadoccio on behalf of the volatile rule of the German Emperors. I had never thought of it in a liturgical context, with Catholics actually praying for it to be expanded and inaugurate universal peace on earth.
It strikes me almost as a sort of Christian answer to the Islamic ‘Caliphate’.
Dont get me wrong, the Holy Roman Empire was a fascinating quasi-federal political structure (a bit like the modern EU) that bound central Europe together in a loose, porous union prefixed upon a common Christian cultural matrix…but did Catholics really expect it to become ‘universalised’ as the prayers seem to infer?