Final answer:
The description fits the history of the Songhai Empire, whose internal conflict and defeat at the Battle of Tondibi led to its decline and vulnerability to invasions.
Step-by-step explanation:
Describing a large scale battle that challenged the rule of a new king and resulted in the military weakening, subsequently making it vulnerable to an invading force from the north, fits the history of the Songhai Empire. After internal conflict and civil war, the Songhai suffered a significant military defeat at the Battle of Tondibi in 1591 against the Moroccan army. This event critically weakened the empire and led to its decline, making it unable to resist further invasions and internal turmoil, illustrating the impact of internal conflicts on a powerful state's ability to maintain its sovereignty.