I have given some thought to the question you raise. My response is this is not the mad world of 1914 when alliances were meaningful, honored; almost; and deadly consequences resulted.
Treaties are a statement of intent, not legally binding. This was even more true in ancient days than is true today, but I am sure everyone remembers the line America drew in the sand over Syria's use of chemical weapons, and the US said, 'never mind.' I counted, from memory not research, the number of alliances not honored in Roman times, I got to sixteen.
The alliance system in all AGEOD games which use such are not binding, instead the typically allow a 'free' declaration of war.