16 Mahajanapadas - ancient history of India

Mahajanapadas
Budhhist literature AnguttaraNikaya gives a list of 16 great kingdoms or Mahajanapadas in the begining of 6th century B.C. Major reason of the formation of Janapadas was use of Iron tools for agricultural & military purposes. Over a period of time small or weak kingdoms either submitted to stronger rulers or got eliminated.
Finally in 6th century BC
 Vatsa
 Avanti
 Magadha
 Kosala were only 4 major kingdoms survived:

Vatsa
The Vatsas or Vamsas are called to be a branch of the Kurus. The Vatsa or Vamsa country corresponded with the territory of modern Allahabad in Uttar Pradesh. It had a monarchical form of government with its capital at Kaushambi.
Udayana was the ruler of Vatsa in the 7th century BCE. He was very powerful, warlike and fond of hunting. Initially king Udayana was opposed to Buddhism, but later became a follower of Buddha and made Buddhism the state religion. Kaushambi was a very prosperous city where a large number of wealthy merchants resided. It was the most important centerport of goods and passengers from the north-west and south.
Avanti
The country of the Avantis was one of the important kingdom of western India and was one of the four great monarchies in India in the post era of Mahavira and Buddha, the other three being Kosala, VatsaandMagadha. Avanti was divided into north and south by the river Narmada. Initially, Mahishmati (Mahissati) was the capital of Southern Avanti, and Ujjain (Sanskrit: Ujjayini) was of northern Avanti, but at the times of Mahavira and Buddha, Ujjaini was the capital of integrated Avanti. The country of Avanti roughly corresponded to modern Malwa, Nimar and adjoining parts of today's Madhya Pradesh.
Both Mahishmati and Ujjaini stood on the southern high road called Dakshinapatha which extended from Rajagriha to Pratishthana (modern Paithan). Avanti was an important center of Buddhism.King NandiVardhana of Avanti was defeated by king Shishunaga of Magadha. Avanti later became part of the Magadhan empires.
Magadha
The Magadha was one of the most prominent and prosperous of mahajanapadas.
The capital city Pataliputra present day Patna, Bihar was situated on the confluence of major rivers like the Ganga, Son, Punpun and Gandak.
The alluvial plains of this region and its proximity to the copper and iron rich areas of Bihar and Jharkhand helped the kingdom to develop good quality weapons and support the agrarian economy.
Its location at the center of the highways of trade of those days contributed to its wealth.
All these factors helped Magadha to emerge as the most prosperous state of that period.
The kingdom of the Magadha’s roughly corresponded to the modern districts of Patna and Gaya in southern Bihar and parts of Bengal in the east.
The capital city of Pataliputra was bound in the north by the river Ganges, in the east by the river Champa, in the south by the Vindhya Mountains and in the west by the river Sona.
During Buddha's time its boundaries included Amga. Its earliest capital was Girivraja or Rajagaha (modern Rajgir in the Nalanda district of Bihar).
The other names for the city were Magadhapura, Brihadrathapura, Vasumati, Kushagrapura and Bimbisarapuri.
It was an active center of Jainism in ancient times. The first Buddhist Council was held in Rajagaha in the Vaibhara Hills. Later on, Pataliputra became the capital of Magadha.
Kosala
The country of Kosala was located to the north-west of Magadha, with its capital at Ayodhya.
Its territory corresponded to the modern Awadh (or Oudh) in Central and Eastern Uttar Pradesh.
It had the river Ganges for its southern, the river Gandak for its eastern, and the Himalaya mountains for its northern boundary. It finds mention as the center of Vedic Dharma.
Its kings allied with the Devatas in various wars against the Daityas, Rakshas, and Asuras.
Kosala and Ayodhya hold a central place in the Hindu scriptures, Itihas, and Purana.
Raghu vansha-Ikshvaku vansha was the longest continuous dynasty; Lord Rama was a king in this dynasty. Other great kings were Prithu, Harishchandra, and Dilip, who are each mentioned in different Puranas, Ramayan, and Mahabharat. According to these texts, Kosala was the most powerful and biggest kingdom ever in the recorded history. Later, the kingdom was ruled by the famous king Prasenajit during the era of Mahavira and Buddha, followed by his son Vidudabha (Virudhaka). King Prasenajit was highly educated. His position was further improved by a matrimonial alliance with Magadha: his sister was married to Bimbisara and part of Kasi was given as dowry. There was, however, a struggle for supremacy between king Pasenadi (Prasenajit) and king Ajatashatru of Magadha which was finally settled once the confederation of Lichchavis became aligned with Magadha. Kosala was ultimately merged into Magadha when Vidudabha was Kosala's ruler. Ayodhya, Saketa, Banaras, and Sravasti were the chief cities of Kosala.