I'm pretty sure no one in history had ever said, "Let's be allies for ten years." I realize this is to make an alliance victory tougher, but what was wrong with the way it worked in GalCiv 2? You know, occasionally an event would pop up - like a government coup or an assassination - that could change the dynamics of a relationship.
Throughout history, alliances often resulted from some event. In addition to a negotiated alliance, an alliance could also spawn from an occasional event that involves a "close" race. It could even be a series of events. Maybe an alliance forged in this manner cannot be broken because it creates bond much stronger than a negotiated "alliance of convenience".
I just feel the alliance thing could be implemented a little better.