Final answer:
Succession disputes and power struggles within royal families were a common source of instability among thirteenth-century Christian and Islamic monarchies, as evidenced by the Mamluk Sultanate and the collapse of the Caliphate of Cordoba.
Step-by-step explanation:
A common source of instability among both Christian and Islamic monarchies in the thirteenth century was succession disputes and struggles for power within royal families. The Mamluk Sultanate, for instance, faced such instability because succession did not typically descend from a sultan to his sons and instead, the army usually chose the new sultan. In Christian Europe, the absence of centralized government and the reliance on oaths of loyalty also presented challenges to political stability. Infighting, such as the collapse of the Caliphate of Cordoba due to succession complications after the death of al-Mansur, and the fragmentation among Islamic rulers also exemplified these power struggles. This instability was not linked to an overabundance of resources, stable political alliances, or dominance in maritime trade routes during this period.