| Conflict: | Magadha–Kosala War |
| Partof: | Wars involving Magadha |
| Date: | Late 5th century BCE |
| Place: | Modern-day eastern part of Uttar Pradesh, India |
| Territory: | Magadhī annexation of the Kosala & Kasi |
| Result: | Magadhī victory |
| Combatant1: | Magadha |
| Combatant2: | Kosala |
| Commander1: | Ajatashatru |
| Commander2: | Viḍūḍabha |
| Strength1: | Unknown |
| Strength2: | Unknown |
| Casualties1: | Less |
| Casualties2: | Heavy |
The Magadha–Kosala War was a military conflict between the Haryanka dynasty of Magadha and the neighbouring Kosala kingdom. The conflict is mentioned in ancient Indian literature of Hindus, Buddhist and Jain traditions. The conflict ended in defeat for the Kosala and the Māgadhīs annexing their territory, including Kashi.
See also: Viḍūḍabha and Pasenadi.
Viḍūḍabha (also known as Virudhaka) was the king of Kosala, son of Pasenadi (also known as Prasenajit). After the Buddha's death heinvaded the Sakya and Koliya republics, seeking to conquer their territories because they had once been part of Kosala. Viḍūḍabha finally triumphed over the Sakyas and Koliyas and annexed their state after a long war with massive loss of lives on both sides. Details of this war were exaggerated by later Buddhist accounts.
In actuality, Viḍūḍabha's invasion of Sakya might instead have had similar motivations to the Māgadhī King Ajātasattu's conquest of the Vajjika League because he was the son of a Vajjika princess and was therefore interested in the territory of his mother's homeland.
The heavy loss of life incurred by Kosala during its conquest of Sakya weakened it significantly enough that it was itself was soon annexed by its eastern neighbour, the Kingdom of Magadha, and Viḍūḍabha was defeated and killed by the Māgadhī King Ajātasattu.
Ajatashatru followed policies of conquest and expansion. He defeated his neighbouring rivals including the King of Kosala; his brothers, at odds with him, went to Kashi, which had been given to Bimbisara as dowry and led to a war between Magadha and Kosala. Ajatashatru occupied Kasi and Kosala and captured the other smaller kingdoms. Magadha under Ajatashatru became the most powerful kingdom in North India.
Under the reign of Mahapadma Nanda of Magadha, Kosala rebelled but the rebellion was put down and Kosala declined forever.