The ai seems to betray the moment the possibility of an easy victory appears. I have ceasefires and possibly alliances that lasted less than a turn. The ai would seek peace and two moves of its units later, it lays siege on a city. I treat ceasefires and alliances with sides that have parts of my territory as objectives as uneventful warfare, a cold war. Dealing with the ERE as the WRE is an instance of this.