Plushie said:
'Citizenship' is hardly relegated to Republican institutions. It merely describes a member of a certain state with certain rights and obligations above those of a non-citizen. Citizens could be taxed at a different rate, could be eligible for certain forms of military service and officer ship, could hold certain offices of state, etc.
Frankly, I think you need to prove to me this wild claim that citizenship means republic.
A certain "state"?

You mean a certain "city". It's in the etymology - "cives" from "civitas", city.
"Citizens" are those free men of the city endowed with civil and political rights. By that I mean, they necessarily participate in the political life of the city, are eligible to vote and stand for office (
suffragium et honores). The status of "citizen" is intimately tied with a republican arrangement.
There are other classes of people - "freeborn men" - who are may reside within the jurisdiction of the city and be protected by its laws, indeed, enjoy all the privileges of citizens except one - political rights. But these aren't "citizens". They were called
latini (if Italian) or
peregrini (if in the provinces). They were not
cives, they did not have the
status civitatis, defined by its entitlement to political participation.
And then there are a whole class of others - "freedmen" (ex-slaves) and slaves naturally, who had no autonomous protection of the law.
Prior to the Caesars, Romans took the definition of citizenship very seriously. Even citizens outside the city walls of Rome were not allowed to exercise those political rights except inside the city walls.
But you're right. Citizenship was degraded and debased by the caesars, and the distinction among free men between citizens, latins and pilgrims, diminished precisely because special republican basis of "citizens" was undermined by their tyranny. The caesars were happy enough to make anyone a citizen, with full rights to political participation, since it didn't matter - the practical ability to exercise that right was null. Everyone was,
de facto latins/pilgrims. The "extension" of citizenship (notably Caracalla's decree) was a meaningless exercise since the label conferred no more practical rights of suffragium and honores than you already had as a latin/pilgrim. But
notionally you are now a
citizen of Rome (not a citizen of the "empire"'; imperium =/= civitas) and endowed with
republican political rights.
That said, let me address one thing you mentioned: the subjects of the Byzantine empire were, funny enough,
not "citizens", in fact or in law. Nowhere in the Justinian code or the institutes do we find the status of "citizen" defined or distinguished. Everyone was either "subjects of Caesar" or "slaves" (although note: not before that). So I suppose you are correct in asserting that it was a monarchy, that SPQR became meaningless in the east.
But that is hardly encouraging. For it also makes it glaringly obvious that the Byzantines were no longer Romans in any sense of the word. For by obliterating citizenship, they had severed the ultimate connection, the only string of continuity to the Roman polity.
