Break your word and the AI remembers for a very long time. For years responses to your diplomatic suggestions will be met with talk about your word not being trusted. Attacking a neutral faction might likewise make factions wary of accepting an alliance?

Ive found alliances easier to make with factions that do not border yours and have a common enemy. That said I want Britian to ally with me so we fight a common enemy, Gaul, that separates us. Britians have declined ..although I have yet to offer a load of cash to sweeten the deal.

If you leave a region bordering an ally with weak garrisons it makes a tempting target. Too good to miss perhaps?