The secret to the stability and cohesion of Ancient Egypt was religion. When people share a core set of beliefs, they are more likely to identify as one. That was true for the first civilizations, it's just as true today.

Examine an ancient civilization unlike any other, that of the Indus Valley. Rather than imposing order through war or religion, it relied on the free flow of trade. The exchange of goods promoted wealth, co-operation and trust.