Well this is the thing. Allying with Spain leads to total stability, since all of the would be enemies of Spain and Byzantia are reduced to fifth rate powers. Total stability therefore leads to Stagnation or to Betrayal. Stagnation because the remaining powers have no way of ever challanging the alliance, meaning no one tries, meaning the game stagnates and dies...Since there is no mechanism for the mega-empires falling apart, as happened to the various Empires in history. Betrayal because, without any enemies to fight, the alliance has no point. So, Byzantia signs up to temporary alliance with a more powerful nation, by burning all of its bridges with any potential ally when the alliance expires. If Byzantia had betrayed us by not launching an attack and instead launching a full attack across Europe, then that would have made sense...We would have defeated/weakened your main competitor while you harvested our lands... But, end of the day, you chose to come second...