strong alliances

[Aemilius Paulus]

Also, never let your own cities that are close to their empire become thinly-garrisoned. I noticed in three campaigns that the moment my ally turns on me happened exactly after I let one of the border cities almost defenceless. It seems the AI evaluates the success probability of a venture, with various factors stacked against each other, and when it decides the military venture is worth more than the alliance & other factors, it goes on with the venture.

To test this, I loaded one of my savegames a turn before they attacked me (my Romani campaign, Saba as the long-time allies, and almost-empty Alexandria as their target city), but this time heavily garrisoning the previously almost-empty border city. I was correct; they did to attack me. In my current campaign as Eperios, I spotted three Pontic (who are my allies) stacks moving for two turns in a row against my Byzantion, which had only one levy unit as a garrison. So I moved an adjacent army into the city, and the next turn, the stacks of Pontic soldiers turned back and dispersed .

EDIT: BTW, in all three campaigns I have given a gift of 500 mnai per turn - and they still attacked me (M difficulty for the first two, and VH for the current Eperios campaign).