to prominent warriors. And writing, when it existed, was often used for kings to boast of military conquests or victories. Writing, when used by the common man, also tended to record such events, as major battles and conquests constituted major events that many would have considered worthy of recording either in an epic such as the Homeric writings pertaining to the Trojan War, or even personal writings. Indeed, the earliest stories center on warfare, as war was both a common and dramatic aspect of life; the witnessing of a major battle involving many thousands of soldiers would be quite a spectacle, even today, and thus considered worthy both of being recorded in song and art, but also in realistic histories, as well as being a central element in a fictional work. Lastly, as nation states evolved and empires grew, the increased need for order and efficiency lead to an increase in the number of records and writings. Officials and armies would have good reason for keeping detailed records and accounts involving any and all things concerning a matter such as warfare that, in the words of Sun Tzu, was "a matter of vital importance to the state". For all these reasons, military history comprises a large part of ancient history. Notable militaries in the ancient world included the Egyptians, Assyrians, Babylonians, Persians, Ancient Greeks (notably the Spartans and Macedonians), Kushites, Indians (notably the Magadhas, Gangaridais, Gandharas and Cholas), Early Imperial Chinese (notably the Qin and Han Dynasties), Xiongnu Confederation, Ancient Romans, and Carthaginians. The fertile crescent of Mesopotamia was the center of several prehistoric conquests. Mesopotamia was conquered by the Sumerians, Akkadians, Babylonians, Assyrians and Persians. Iranians were the first nation to introduce cavalry into their army. Egypt began growing as an ancient power, but eventually fell to the Libyans, Nubians, Assyrians, Persians, Greeks, Romans, Byzantines and Arabs. The earliest recorded battle in India was the Battle of the Ten Kings. The Indian epics Mahabharata and Ramayana are centered on conflicts and refer to military formations, theories of warfare and esoteric weaponry. Chanakya's Arthashastra contains a detailed study on ancient warfare, including topics on espionage and war elephants. Alexander the Great invaded Northwestern India and defeated King Porus in the Battle of the Hydaspes River. The same region was soon re conquered by Chandragupta Maurya after defeating the Macedonians and Seleucids. He also went on to conquer the Nanda Empire