- Of the 16 Mahajanapadas, four most powerful were – Kosala, Vajji confederacy and Magadha in the middle Ganga Valley and Avanti in western Malwa.
- After some time, Avanti and Magadha remained the most powerful Mahajanapadas. Avanti emerged as a challenger to the dominance of Magadha and annexed Vatsas of Kaushambi and planned to invade Magadha. Fearing the invasion Ajatashatru started building fortification of Rajgir. However, the invasion did not take place.
- Magadha became the most powerful Mahajanapadas in the 6th and 4th centuries under the Nandas.
Reasons for Emergence of Magadha as the most Powerful Mahajanapadas were:
- Rich soil: Magadha was a region where agriculture was especially productive due to fertile alluvial lands. The heavy overgrowth could be cleared using iron implements. Various varieties of paddy were grown in the region. This enabled farmers to produce surplus which also augmented state taxes.
- Ease of transportation: Early Magadhan capital of Rajgir and later capital Pataliputra were located close to the Ganga River and its tributaries. This enabled Magadha to use the river as communication route by the army, other administrative purposes and commercial purposes. Location of Pataliputra made its position like a water fort (Jala durga). The location also enables Magadhan kingdom to command over the Uttarapatha which connected the entire northern India.
- Availability of iron and other minerals: Iron mines were available in proximity in Jharkhand region and provided resources for tools and weapons. The region was also located in rich copper fields. Avanti too had access to rich iron ore mines in eastern Madhya Pradesh.
- Availability of timber: Magadha was surrounded by abundant forests which were rich sources of forest and timber. Timber was extensively used in construction of houses in Magadha region as noted by Megasthenes. Timber could also be used to make boats to dominate the Ganga River militarily.
- Availability of elephants: Elephants were important components of army of these times and were abundantly available in the vicinity of Magadha. Elephants had an advantage over horses and chariots because they could be used to march across marshy land and areas with no roads or other means of transport.
- Location of Rajgir (capital of Magadha): Rajgir: Rajgir was also known as Rajagriha or Girivraja. It was located near the south bank of Ganga River and surrounded by seven hills which made it very secure and impregnable.
- Unique social set up in Magadha: The societal set up of Magadha unorthodox in character as it was mixture of Vedic and non-Vedic people as compared to other Mahajanapadas which were dominated by orthodox Vedic people. This made Magadha to be more receptive to expansionist policies of its rulers.
- Superior military Technology: Magadha had a superior military technology due to use of two weapons, mahashilakantaka (a large sized catapult used for hurling rocks) and rathamusala (a chariot fitted with a mace which caused terrific destruction when driven through the enemy ranks).
