Kurukshetra war
This was the most important event in Dwapara yuga. 18 Akshauhinis got destroyed in this war. Only 11 people survived the war.
Khandava war
For the sake of Agni, Arjuna and Krishna fought the devas, nagas, Rakshasas and Gandharvas in Khandava. Arjuna baffled all attempts by Indra to stop the fire in the forest.
Narakasura war
The wicked demon Narakasura of Pragjyotishapura terrorised the 3 worlds and imprisoned 16,000 women. Lord Krishna, riding on Garuda destroyed 800,000 Danavas in the city and slayed Naraka along with Mura and Nishunda.
Banasura war
When Krishna's grandson Aniruddha was imprisoned by the asura Bana, Krishna, Balarama and Pradyumna arrived in the city of Shonitapura to liberate him. Here Krishna fought with Skanda, Shiva and the bhoota ganas. Krishna sliced down the 1000 hands of Bana but spared his life on Shiva’s request.
Nivatakavacha, Kalakeya, pouloma war
While training in Swarga, Arjuna for the sake of Indra single handedly destroyed the powerful Nivatakavachas in battle. Then savyasachi consumed the Kalakeyas and Poulomas of Hiranyapura with his Roudrastra.
Virat war
One of the most overwhelming victories in history. Arjuna on a single chariot crushed the entire Kaurava army in battle. Bhishma, Drona and Karna could not match his valour.
Jarasandha's Invasion of Mathura
After Kamsa's death, Jarasandha invaded Mathura 18 times with an army of 20 Akshauhinis. That is larger than the combined Pandava and Kaurava army in Kurukshetra. Each time Krishna and Balarama defeated Jarasandha but kept sparing his life. Finally Krishna shifted the Yadavas to Dwaraka.
There are several other lesser wars like Pandava Digvijaya, Shalva - Krishna war, Bhishma vs Parashurama, Arjuna Digvijaya during Ashwamedha etc.