Thanks :) I'm more of a returning member - been probably a decade or so since I've last posted!

In the past year or two I've gotten a much better handle on the diplomatic aspects of this game (mostly via the Fourth Age mod, which I work on). The absolute highlight of that research has been an understanding of how to acquire protectorates, which can provide you with a useful buffer zone as well as a ton of cash (although in vanilla RTW I've noticed protectorates tend to pay you very little, if anything; probably a result of the lower incomes on the vanilla map compared to the mod?). The next most useful tidbit has been an understanding of how to gift provinces to the AI, which can also result in buffer zones.

I should note that these things are mostly useful to players who prefer to keep lots of factions 'alive' throughout the campaign. A player who just wants to 'win' quickly and kill 'em all won't have as much fun with these maneuvers, which take time and often lots of cash to make happen.

In the case of Salona, the calculation there was about balance of power in the region. If I let the province rebel, it's surrounded by the Brutii, and I'm sure they'll get it back soon.

At the time, I wasn't yet at war with Thrace. Thrace held 5 settlements (Campus Getae, Tylis, Byzantium, Bylazora, Thessalonica). Macedon was essentially gone, banished to Africa. Greeks held Larissa and everything south of it. Brutii held Thermon and Apollonia in the region.

Knowing that Thrace is shaping up to be the major power (and would attack me soon), I wanted to use the Greeks as a counter. Propping them up with an extra province and some cash would - hopefully! - allow them to push back against the Brutii, and prevent Thrace from gaining more land in Greece.

Of course, it's somewhat risky, for the reasons you state: it's costly, and it will strengthen a faction that is sure to be an enemy down the road. But things are currently pretty manageable, fortunately. I've managed to reduce Thrace from a 5-province faction down to just 3, with 2 of them (Bylazora, Byzantium) currently under siege. They no longer pose any real threat, so after taking those 2 settlements the time may be right for a ceasefire, which will allow me to turn my full attention to the Romans.

As for the Greeks, they have so far managed to retain Salona, and have attempted (but failed) to take back Thermon. That's fine - a stalemate, with slight momentum on the Greek side, is better IMO than a Brutii conquest of Greece.