What was the main cause for the outbreak of the Second Sudanese Civil War as noted in the nonfiction book, A Long Walk to Water?

The government, which is located in the northern part of the country, has decided that Sudan is a Muslim nation, and that everyone must convert to Islam. In the southern part of the country, where Salva's village is located, many different religions are practiced and many of the people there do not want to convert to Islam. Thus, the rebels began fighting for their independence.