I moved the Roman history stuff to the bottom, and will answer the game
related stuff 1st. So folks only interested in that should read that, then
they can, if they choose, skip the rest (but you might get a campaign idea
if you read on, you never know...)

O.K. In any culture or civilization there will be forces pulling
things together and forces pulling things apart. Some of these will
be technological, some geographical, some cultural. This is on topic!
Empire-building is a going Birthright concern!

IMO, making game world nations dynamic is one of the hardist things to do;
tendency is to have them sit their, inert, waiting for the P.C.s to contact
them in some way, then have them respond haphazardly. But, if you want a
campaign world that seems alive, these places should develop in ways, even
if the P.C.s never intend to contact them; heck, might make the P.C.s
wonder, at least...lots of time some natural development in a nation,
having nothing to do with the players, got 'em all stired up. Players are
naturally paranoids...

Birthright application is: How do you treat and assimilate new
provinces? As conquered territory? As second class citizens? How do
you persuade them to raise reliable units for you, especially against
their former lord?

That's a good q.. I had a whole bunch of stuff written up, then my computer
locked up on me, and I lost it all. Suffice to say that their are many ways
to gain at least the aquiesense (spelling again) of conquered territory,
not all of them "good". The Assyrians and (to a lesser extent) the Mongols
relied on fear in building and maintaining their empires (I know, some of
you are shaking your head saying "all imperialism is founded on fear" well,
lets set asside that for a moment, and the varing degrees to which it is
mustered and employed, etc. Remember, this is a fantasy game, so if a
Paladin conquers a neighboring province; say he has a good reason [evil
dudes held it before, and he's liberating it from the diabolical scourge],
he isn't going to use Assyrian methods, we hope). But, lets say you don't
want to encourage your players to build their empires with a pyramid of
human skulls. This is still good to know because the villians of the
campaign world (say, the Gorgon, or the Magian) use these methods, and, by
all accounts, run pretty efficient realms (a threat to all their neighbors,
which is efficient enough for them). The Magian, for example, will sugar
coat this; he'll try to make it attractive to obey him (hey, he brought
order to Pripyett, and was welcomed in at least one of the provinces). His
ruthlessness will be more on the order of Machievellie's advice to the
Prince (if you're gonna kill a bunch of people, do it all at once; the
people will forget this a lot quicker than if you kill people over a long
streach of time...) rather than constant beheadings.
For P.C.s, though. Well, from the rules it appears that when you conquer a
P., you can pretty much assure it's loyalty if you keep taxes low (at least
until you get a firm grip on the place with law holdings) and don't opress
people to much. But, from the backround, I'd say this only applies when
you're conquring a province of the same culture as yours (say, a Rjurik
king conquring a province inhabited by Rjurik), and if the previous ruler
wasn't very loved (he doesn't have to be hated; pesant's will pretty much
not care who their lord is, they're pesants no matter what and life is
gonna be pretty much the same [meet the new boss, same as the old boss...],
as long as that ruler wasn't especially loved for some reason or another).
But if that same king conquers a province of Dhoesone, say (Anurian
culture), then you've got all kinds of other issues. If he keeps taxes low
(say, lower than back in the homeland), then his own subjects will get PO'd
("hey, we paid high taxes, died in the army, and you're giving the
barbarian devils a better deal than your own people!) And their will be
clashes because you don't speak the same language, use the same laws, etc
(meet the new boss...NOT the same as the old boss, and we pesants don't
like change, at least when it involves rule by these barbarian devils!) If
by coincidence the predomidant religion in both your land and the conquered
province is the same, that should be a plus, if you bring some benifit to
the conquered province (say, before they were plagued by bandits and
monsters, but your troops drove 'em off, and brought security to the
province, or the province was poor, and you build roads and bring in trade
and increase prosperity), then the populace will be grateful (for about a
week or so; gratitude to foreigners doesn't seem to last too long. sigh.)
Best thing is to bring in some of your own people (a "colony" in the Roman
sense). They, along with cooperative natives (or, depending on your
perspective, collaborators and quislings), should keep the place somewhat
quiet. This "colonization" might require Ruling the province up a level, or
some simmilar deal worked out between the player and the DM. One thing for
thouse inclined to rule by fear, at least a little, is to Pilliage the
place when they 1st move in (thus killing or driving off a portion of the
natives), then rule it up, presumably with new colonists comming from their
heartlands. Remember, the Anurians were able to manage to rule most of
Cerilia for a couple of centuries. While they were probably never popular
overlords, the whole place wasn't in constant rebellion, either, and no
doubt they were able to muster some auxillary troops from the conquered
areas, at least (but probably couldn't trust 'em too much). Best thing to
do with troops mustered in one area is to garrison them in another area
(the old AustroHungarians did this, and managed to keep their cobbled
together realm from falling apart until *everything* fell apart in the big
one). God, this is long enough.

Oh, wait; one more issue I just can't ignore. Conquring areas inhabited by
another race (say, humans conquering a dwarven province). No, just doesn't
seem to happen. Best you can do is 1) Conquer the provinces (like that
Ghuralli dude did to Lluabraight's provinces of Caer Cwnnar and Rhuiddar)
while driving out the inhabitants or 2) negotiating some kind of alliance
or Patron-Client [state] relationship with them, a kind of international
protection racket, or a true (!) alliance of friendship (they be your
friend, and in return you help protect them against their enemies). The
latter requires that they need, or think they need some help, and they can
trust you to provide it. Then you've got to negotiate a friendly deal. You
might get some Dwarven units to serve in your army that way (cool!) I
doubt, however, you could reach an arangement like this with an Elven land
(but, strange things happen, especially when PCs are involved, or even if
they are not; see that Player's Secret's of Tuarheval. Ever thought
something like *that* could be possible?...)

I'm too tried to deal thoughtfully with this, but a very preliminary
thought is that Rome got is start as a major state by the combined
military improvements of the maniples of the legion, and by the
political genius of incorporating conquered cities as citizens, and a
source for new legions, rather than as conquered and suitable for
pillaging territories.

well, mmmnnh, kind of; Rome got it's start 'cause it found a way to coopt
the local Latin city states, without granting them citizenship in Rome
(till after the Social Wars, by which point the Republic was pretty much a
shot concept). Rome expanded, at first at least, 'cause they were better
able to employ their manpower, and they wouldn't give up if they
encountered setbacks (unlike the Diodachi and the Carthaginians, for whom
if war wasn't profitable, it wasn't worth continuing, for the most part).
Rome managed to attract the loyalty of its allys and conquered areas
because they gave a slightly better deal than thouse places were likely to
get from someone else (thus, when Hanibal came into Italy, expecting to get
support from the "opressed" Greek and Latin etc. cities and tribes, he got
little; because they didn't expect to be treated better by the
Carthaginians, and probably because these places figured that if they
opened their gates, H's mostly mercenary, mostly barbarian army, would put
'em to the sack regardless of H's promises, even if *he* meant to keep
them). For the most part, Rome aquired its empire in a fit of
absentmindedness [like that phrase? I stole it myself], and kept it
together by cobbling together policies that, usually just in time, managed
to persuade folks to prefer Roman rule to the alternatives (which usually
*didn't* include native rule; usually it was Rome or some other foreign
invader. The option wasn't national liberation). Greek city states found
Roman arbitration, for example, preferable to arbitration by that city over
the next hill, because that city over the next hill was a long time rival,
and Rome was distant, the Senate usually impressive, and, well, when thouse
Romans set their mind to something, the best thing to do was to say ok,
'cause they made saying no so darn hard.) Lots of places (the Selucids,
much of Anatolia [oh, that's Asia Minor or Turkey to you ignorati]) just
gradually faded away; Rome didn't really want to *rule* these places, but
trouble kept brewing up, so they kept sniping away at them. Eventually some
of these places in effect just threw in the towel, and their kings willed
the country to Rome on their death. This isn't really a good option for
PCs, though, 'cause it takes so darn long. For PCs the best methods are the
(IMO) less effective and welcome ones used by Caesar (I.E., slaughter all
the celts you can find in a few short years, grab their land and what
little cash they've got--sure, they're poor, but their's so darn many of
'em, you can make yourself rich anyhow), kill off (or let them kill
themselves off a la Crassus) your rivals, grab rich neighboring lands ruled
by women (Egypt, say) who you can bed (can I say 'bed' in this post? It
happened, you know) even if it makes you unpopular back home, and try to
get yourself crowned king. Oh, make sure all your lieutenants are
loyal...to late. Well, perhaps that young Octavianus can do better. Doesn't
seem to be much, though, does he? ::sigh:: you work with what you've