There are several references about the contribution of the Anarta Kingdom to various events in Mahabharata.

Anarta as mentioned in Mahabharata

In the Sixth Book, Chapter 9 of Mahabharata (MBh. 6.9), the Anartas, or people of Anarta region, are mentioned along with several other: “the Pundras, the Bhargas, the Kiratas, the Sudeshnas, the Yamunas, the Sakas, the Nishadhas, the Anartas, the Niritas, the Durgalas, the Pratimasyas, the Kuntalas, and the Kusalas”.

Anarta as a safe haven and training ground for the sons of Pandavas

In the Third Book, Chapter 182 of Mahabharata (MBh. 3.182), it is described that when the Pandavas were exiled by Kauravas from their own kingdom, the five sons of Pandavas, born to Draupadi, too were sent away from Hastinapur. First, they went to the Panchala Kingdom, which was ruled by their maternal grandfather Drupada, and then, to the Anarta Kingdom. In the Anarta Kingdom they learned the art of warfare from the eminent Anarta warriors.

Krishna narrates to Draupadi how her sons spent their time in Anarta in the following words:
‘Those sons of yours, are devoted to the study of the science of arms, are well-behaved and conduct themselves on the pattern of their righteous friends. Your father and your uterine brothers proffer them a kingdom and territories, but the boys find no joy in the house of Drupada, or in that of their maternal uncles. Safely proceeding to the land of the Anartas, they take the greatest delight in the study of the science of arms. Your sons enter the city of Vrishnis and take an immediate liking to the people there. As you would direct them to conduct themselves, or as the respected Kunti would do, so does Subhadra (their step-mother) direct them in a watchful way. Perhaps, she is still more careful of them. As Pradyumna is the preceptor of Aniruddha, of Abhimanyu, of Sunitha, and of Bhanu; so he is the preceptor and the refuge of your sons also! And a good preceptor, would increasingly give them lessons in the wielding of maces and swords and bucklers, in missiles and in the arts of driving chariots and of riding horses, being valiant. An he, Pradyumna, the son of Rukmini, having bestowed a very good training upon them, and having taught them the art of using various weapons in a proper way, takes satisfaction at the valorous deeds of your sons., an of Abhimanyu. O daughter of Drupada! And when your son goes out, in pursuit of (out-door) sports, each one of them is followed thither by chariots and horses and vehicles and elephents.’

Later, Krishna tells Yudhisthira, the exiled Pandava king, who are those valiant commanders and warriors from Anarta ready to fight on Pandavas’ side. They consist of Satwata, Dasarha, Kukura, Adhaka, Bhoja, Vrishni abd Madhu tribes. They are prepared to overthrow the enemies of Pandavas. Bala Rama, with plough as his weapon, will lead warriors consisting of bowmen, horsemen, foot-soldiers, chariots, and elephants.

Kunti in Anarta

The Fifth Book, Chapter 83 of Mahabharata (MBh 5.83) mentions that during the period of exile of Pandavas from Hastinapur their mother Kunti too stayed in Anarta for some time.

Alliance of Anarta for the Great War

In the Fifth Book, Chapter Seven of Mahabharata (MBh. 5.7) one finds detailed account of the fruits of efforts made by both Duyordhana and Arjuna while they visited the City of Anarta to seek alliance of the Anarta warriors for the impending war between the Kauravas and the Pandavas.

Some Anarta warriors opted to join with the Kaurava forces, while some with the Pandava forces. The Yadava king Vasudeva Krishna himself allied with the Pandavas. He promised not to take any weapons in the war, but participate in it as a diplomat, an ambassador of peace, an adviser on war-strategy, and as the guide and chariot driver of Arjuna. However, he gave his army called the Narayanas, which was made of a large number of very capable fighters, to the Kaurava chief Duryodhana.

Balarama, brother of Krishna, wished to help Duryodhana and fight on the side of the Kaurava army. But, since that would involve fighting against his own brother Krishna, who had chosen to be the chariot-driver of the Pandava leader Arjuna, Balarama took a neutral position. He decided not to participate in the war at all and went on a pilgrimage over the river Saraswati.

Anarta warriors annihilated

Kritavarma, the Bhoja Yadava hero, joined his forces numbering an Akshauhini of troops with the Kaurava army. Against that, Satyaki, another great Anarta commander also having an akshauhini troops, sided with the Pandavas. They faught against one another and were involved in a fierce one to one duel. (MBh. 9.21)

However, both these Anarta generals survived the Kurukshetra war. But 36 years later, they were involved in a drunken brawl accusing one another of unfair conduct during the war. In the ensuing melee both of them, along with the rest of Yadavas, were killed as it was ordained by the curse of Gabdhari. (MBh. 16.3)

Thus, the Kurukshetra war proved to be the cause of the extermination of so many Anarta warriors, that the military prowess of the Anarta Kingdom waned afterwards.

Reference for Mahabharata:
Mahabharata of Krishna Dwaipayana Vyasa, translated to English by Kisari Mohan Ganguli, Bharat Press, Calcutta, 1883-97