Byzantine emperors ultimately doomed their empire through a lethal combination of chronic civil wars, aggressive overspending, and reliance on foreign mercenaries. Instead of uniting against external threats, rival rulers routinely bankrupted the state and dismantled crucial frontier defenses to seize the throne. Furthermore, granting vital trading monopolies to rival Italian republics crippled their own economy, while weaponizing Turkish mercenaries in internal disputes opened the door for Ottoman expansion.