Another example of division is the conquest of the Byzantine Empire. Byzantium fought a centuries long de facto 2 front war, which was more than their economy could support. Rather than concentrating on building a strong defense in the south, Constantinople initially tried to reassemble the Roman empire. They invaded distant Spain, Sicily, etc... This alienated potential western allies, and prevented Constantinople from organizing an effective defense in the south.
When the invasion started to capture their territory, they finally paid serious attention to the southern front. They asked for help from the west, but then alienated their allies by demanding all territory captured by westerners be turned over to them. The west refused, and everyone launched attacks that were uncoordinated with each other. The Byzantines sent an army, but one of their generals left the battlefield, and returned to Constantinople to initiate a palace coup. After his half of the army was defeated, the emperor then returned to Constantinople and fought a civil war to regain his throne. This civil war prevented the Byzantines from regrouping to present a credible defense against the southern invasion. They steadily lost ground.
Then the west decided to conquer Byzantium, and sacked the capital. The emperor escaped, and ruled a tiny piece of land between the triumphant crusader army and the southern invasion. The rest of Byzantium was divided up between invading crusader armies. The emperor then fought a long battle to regain his capital, but this once again prevented him from organizing a credible defense against the southern invasion. The southern invasion then moved north, captured Constantinople, and marched on the Balkans beyond it.
Byzantium was strong enough to hold the line in the south, but not strong enough to wage a 2 front war the way they did. A combined European/Byzantine army was strong enough to retake the Middle East, but they chose to endlessly squabble with each other instead. Thus the southern invasion swept all before them, during periods then they were united by a inspiring leader. Ditto for the Mongols. When they fought each other, they were easy game for surrounding nations. When they united under Genghis Khan and his family, they conquered an impressive empire.