The big IF

[Matrim]

#4 Loyal AI: If allied to a certain faction and you come to their aid, especially more than once, they will do the same for you. Or at least, be less likely to declare on you. Something like Europa Universalis's relations meter.

#5 Military Access: Your troops may move into/through allied lands, so long as they do not stay more than, say, 3 turns. This will allow your "coalition" forces a better opportunity to fend off enemies and to bolster offensives. (To balance, if your troops are in allied countries, you cannot declare war/break alliance with them.)