The march of the Magadhan empire during the two centuries preceding the rise of the Mauryas is like the march of the Iranian empire during the same period.
The formation of the largest state in India during this period was the work of several enterprising and ambitious rulers such as Bimbisara, Ajatashatru, and Mahapadma Nanda.
They employed all the means in their power, fair and foul, to enlarge their kingdoms and to strengthen their states. This, however, was not the only reason for the expansion of Magadha.
There were some other important ones. Magadha enjoyed an advantageous geographical position in the age of iron, because the richest iron deposits were situated not far away from Rajgir, the earliest capital of Magadha. The ready availability of the rich iron ores in the neighbourhood enabled the Magadhan princes to equip themselves with effective weapons which were not easily available to their rivals. Iron mines are also located in eastern MP, and were not far from the kingdom of the Avanti with their capital at Ujjain.
Around 500 BC, iron was certainly forged and smelted in Ujjain, and probably the smiths manufactured weapons of good quality. On account of this Avanti proved to be Magadha’s most serious competitor for supremacy in north India, and Magadha took about a hundred years to subjugate Ujjain. Magadha enjoyed certain other advantages. The two capitals of Magadha, the first at Rajgir and the second at Pataliputra, were situated at very strategic points. Rajgir was surrounded by a group of five hills, and so it was impregnable in those days when there was no easy means of storming citadels such as cannons.
In the fifth century BC, the Magadhan princes shifted their capital from Rajgir to Pataliputra, which occupied a pivotal position commanding communications on all sides. Pataliputra was situated at the confluence of the Ganges, the Gandak, and the Son, and a fourth river called the Ghaghra joined the Ganges not far from Pataliputra.
In pre- industrial days, when communications were difficult, the army could move north, west, south, and east by following the courses of the rivers. Also, the position of Patna itself was rendered invulnerable because it was virtually surrounded by rivers. While the Ganges and the Son girdled it on the north and west, the Poonpun girdled it on the south and east. Pataliputra was therefore a true water fort (jaladurga).
Magadha lay at the centre of the mid-Gangetic plains, the Ganges providing a means of both transport and agricultural facilities. As most of the mahajanapadas were located in the Gangetic plains, they could be reached by navigating the rivers.
There was also an abundance of timber as can be seen in the palisades of the sixth century BC found south of Patna. Megasthenes speaks of the wooden walls and houses in Pataliputra. Thus boats could be easily manufactured and they played an important part in promoting the advance of Magadha towards the east and the west.
Similarly, environmental factors conducive to agriculture helped Magadha. The alluvium, once cleared of jungles, proved immensely fertile. Given the heavy rainfall, the area could be made productive even without irrigation. The countryside produced varieties of paddy, which are mentioned in the early Buddhist texts. This area was far more productive than the areas to the west of Allahabad. This naturally enabled the peasants to produce a considerable surplus, which could be mopped up by the rulers in the form of taxes.
The princes of Magadha also benefited from the rise of towns and use of metal money. A Pali text speaks of twenty towns in the age of the Buddha. Most of them were located in the mid-Gangetic plains. They contributed to trade and commerce in north-east India. This enabled the princes to levy tolls on the sale of commodities and accumulate wealth to pay and maintain their army.
Magadha enjoyed a special advantage in military organization. Although the Indian states were well acquainted with the use of horses and chariots, it was Magadha which first used elephants on a large scale in its wars against its neighbours. The eastern part of the country could supply elephants to the princes of Magadha, and we learn from Greek sources that the Nandas maintained 6000 elephants. Elephants could be used to storm fortresses and to March across marshy and other areas lacking roads and other means of transport.
Finally, we may refer to the unorthodox character of Magadhan society. It was inhabited by the Kiratas and Magadhas, who were held in low esteem by the orthodox brahmanas. It however underwent a happy ethnic admixture with the coming of the Vedic people. As it had been recently vedicized, it demonstrated a greater enthusiasm for expansion than the kingdoms that had been brought under the Vedic influence earlier. For all these reasons, Magadha succeeded in defeating the other kingdoms and in founding the first empire in India.