During the Medieval era of Europe, the different religions were introduced, creating a crack of division of the Catholic, and the religion introduced by John Calvin, Predestination. The city states each believed in different religions, creating war in Europe. There were also power struggle among the people. The religious people such as popes and bishops were not serving Jesus godly anymore, but just used as a source of money, wealth, and power. It created corruption in Europe, ending the Medieval era with the start of the Renaissance period.
The Maya organized themselves into small city-states instead of one big empire. The largest was Tikal, which by 750 CE had about 40,000 inhabitants, in specialized occupations and ruled by elites.
The correct answer would be the many different factions in the war. A large number of casualties in the Nigerian civil war or also known as Biafran War, can be attributed to the many different factions in the war with one another.