As Brhannala, Arjuna fought on behalf of Uttara, the son of King Virata, and thus defeated the Kurus in the fight incognito
SB Canto 1
When his inimical cousin Duryodhana was under the clutches of the Gandharvas, he wanted to save him and requested the Gandharvas to release Duryodhana, but the Gandharvas refused, and thus he fought with them and got Duryodhana released. When all the Pāṇḍavas lived incognito, he presented himself in the court of King Virāṭa as a eunuch and was employed as the musical teacher of Uttarā, his future daughter-in-law, and was known in the Virāṭa court as the Bṛhannala. As Bṛhannala, he fought on behalf of Uttara, the son of King Virāṭa, and thus defeated the Kurus in the fight incognito. His secret weapons were safely kept in the custody of a somi tree, and he ordered Uttara to get them back. His identity and his brothers' identity were later on disclosed to Uttara. Droṇācārya was informed of Arjuna's presence in the fight of the Kurus and the Virāṭas. Later, on the Battlefield of Kurukṣetra, Arjuna killed many great generals like Karṇa and others. After the Battle of Kurukṣetra, he punished Aśvatthāmā, who had killed all the five sons of Draupadī. Then all the brothers went to Bhīṣmadeva.
It is due to Arjuna only that the great philosophical discourses of the Bhagavad-gītā were again spoken by the Lord on the Battlefield of Kurukṣetra. His wonderful acts on the Battlefield of Kurukṣetra are vividly described in the Mahābhārata. Arjuna was defeated, however, by his son Babhruvāhana at Maṇipura and fell unconscious when Ulūpī saved him. After the disappearance of Lord Kṛṣṇa, the message was brought by Arjuna to Mahārāja Yudhiṣṭhira. Again, Arjuna visited Dvārakā, and all the widow wives of Lord Kṛṣṇa lamented before him. He took them all in the presence of Vasudeva and pacified all of them. Later on, when Vasudeva passed away, he performed his funeral ceremony in the absence of Kṛṣṇa. While Arjuna was taking all the wives of Kṛṣṇa to Indraprastha, he was attacked on the way, and he could not protect the ladies in his custody. At last, advised by Vyāsadeva, all the brothers headed for Mahāprasthan. On the way, at the request of his brother, he gave up all important weapons as useless, and he dropped them all in the water.