Background: I have played some 200 turns as Rome now on my first long-term playthrough. I have had Macedonia as my only ally since very early in the game, and they have never done anything to break the pact even though I have had seriously under-garrisoned regions neighboring them. I have the entire Illyrian coast down to Laconia in my possession, and Macedonia has had a difficult existence with eastern aggressors east all through my campaign. Now we are both threatened by Getae, who attack Macedonia and attempt to conquer their first region neighboring me, an Eleutheroi region. I do not want to lose my only ally, and I don't want the aggressive Getae neighboring my under-developed and weakly garrisoned Illyrian coast. There are many factions that I need to defend against anyway in despite of having avoided conflict myself unless attacked.
The Getae recently did a series of sieges against said Eleutheroi city and managed to weaken its defenses considerably. They were interrupted by my Macedonian allies, and I decided to seize the moment and besiege the weakened city. I would now like to conquer it and donate it to Mecedonia to strengthen them, keep the Getae at a longer distance beyond a Macedonian buffer zone so I can concentrate on other, quite numerous threats that I face.
Question: Will giving the region to Macedonia help maintain our alliance, or is there some bs game mechanic that sees giving regions as a sign of weakness or some other invitation to break the alliance? If it will make things worse, is there any situation in which giving a region to any faction would be beneficial in any way? Would our alliance benefit from me declaring war on Getae and fighting together?