What leader? Only Carthima has been in more alliances than you have, and only TAB ever really considered you a part of AMEP.
Furthermore, you *offered* Cloud the opportunity to join you in your campaign of aggression against the (at the time and apparently) peaceful MTVS. No promises were expressed by either him or you regarding future conflicts. Maybe they were implied, but implication means about as much as my name.
I don't know the circumstances of Sephi's departure from AMEP, but unless you can give screened quotes of a contract, he was a free agent after he made FotA. Seriously, WHAT LEADER? Bug? TAB? Hurricane? Draze?? Scarf?? Nss4fun???