I think there should be multiple forms of alliances. This could range from aggreements to declare neutrality in a war if you're not the aggressor to being the ally of an alliance. In World War I for example, Turkey was on the side of the Triple Alliance (Germany, Austria-Hungary, and (origanaly) Italy) but wasn't a member. I also think that you should be able to be in multiple alliances. This would be similer to the alliances of unified Germany during the time of Otto von Bismark.