The Scottish traveler James Bruce reported in 1770 that Medri Bahri was a distinct political entity from Abyssinia, noting that the two territories were frequently in conflict. The Bahre-Nagassi ("Kings of the Sea") alternately fought with or against the Abyssinians and the neighbouring Muslim Adal Sultanate depending on the geopolitical circumstances. Medri Bahri was thus part of the Christian resistance against Imam Ahmad ibn Ibrahim al-Ghazi of Adal's forces, but later joined the Adalite states and the Ottoman Empire front against Abyssinia in 1572. That 16th century also marked the arrival of the Ottomans, who began making inroads in the Red Sea area.

Answer this question, if possible (if impossible, reply "unanswerable"): What determined whether The Bahre-Nagassi fought with or against the Abyssinians?
geopolitical circumstances