Unfortunately, in R:TW the AI is, well, let's say, rather stupid when it comes to diplomacy. The only way that an alliance can be forged and generally stuck too is through tributes. The AI, if payed a reasonable tribute, along with an alliance, will generally not be hostile and will, most of the time, leave you alone and shall remain as your allies. Sadly, this is not law and, of course, there always are nasty exceptions.