In the ancient world, what were the advantages and disadvantages of living in a unified country under a single central authority compared to living in a region with separate city-states?
To answer your question, we will compare two civilizations in the ancient world: Mesopotamia and Egypt. By 4000–3000 B.C., Mesopotamia consisted of independent city-states (each with its own ruler), while Egypt flourished around 3050 B.C. as a unified political state.
The Sumerians in Mesopotamia established twelve city-states in their time, including the states of Eridu, Ur, and Uruk. Each state flourished because of agriculture and trade. In time, however, the city-state of Akkad waged war on other city-states because of its desire to control the production of metals. In the ancient world, the invention of bronze led to the creation of powerful and deadly weapons of war as well as formidable agricultural tools.
Because Akkad wanted to acquire metals for its own gain, the city-state unleashed war on its neighbors, thus establishing the world's first empire. The Akkadian empire was eventually overthrown by the Ur III dynasty (2112–2004 B.C.). The Ur dynasty was centrally ruled and did not consist of city-states. However, continual civil war within its borders (actuated by the ambitions of warring lords in the kingdom) led to its eventual fall.
So, while a unified country under a strong central authority has its advantages, it also has its disadvantages. A monarch can marshal a powerful, unified army to fight his kingdom's enemies, but political fractures that lead to civil wars within the kingdom are more difficult to suppress.
Yet, city-states have a major advantage. For example, the Canaanite city-states promoted increased interaction between traders of different nationalities and cultures. This multilingual environment led to a very important development in 1600 B.C.: the creation of the Canaanite alphabet, which became the basis of the Greek and Roman alphabets, which in turn became the basis for the modern Western alphabet system. Eventually, the Canaanite city-states came to be dominated by the Egyptian empire.
While the Egyptian kings wielded powerful armies to protect their people, they were also ruthless leaders. The free common peoples had heavy obligations to the state. During a lull in the planting seasons, the pharaohs required Egyptian laborers to build formidable pyramids; their labor constituted a percentage of their tax payments.
So, the advantages of living in a unified country are that citizens are protected by their kings and increased trade opportunities often lead to important social developments. On the other hand, a major disadvantage of living in a unified country is that political fractures often threaten its social and economic stability.
Source: The Making of the West, Combined Volume: Peoples and Cultures, Volume 1, Lynn Hunt.
The costs and benefits of living in a unified country depended to some degree on who you were in a society. They also depended on the skill with which the rulers of the unified country ruled. In general, though, unified empires provided somewhat more stability and prosperity than city-states did.
The main reason that an empire would be more stable is because there would be fewer potential wars to be fought. Of course, empires did not always have peace. They might come under attack from the outside or they might try to expand their territories through war. However, regions with many city-states had many more potential war-partners. In other words, if there are a lot of different city-states, there are a lot of different combinations of possible wars.
The main reason why an empire would be more prosperous is because an empire would function like a large free trade zone. In a well-run empire, people could trade across long distances without having to worry about being attacked or about having to pay taxes when entering a new city-state. They could trade without having to worry about the various laws of the various city states. More trade would typically lead to more prosperity.
Of course, life in a unified empire could be difficult. Rulers of large empires could be very overbearing. They could impose huge tax burdens. People who had been conquered might have very difficult lives because they were not of the same group as the ruling elites.
Thus, there were positives and negatives to life in a large, unified empire.