The great war described in the epic Mahabharata by Ved Vyasa was fought between cousins Pandava and Kaurava and saw participation from many kingdoms across Bharatavarsha.
Duryodhana the leader of Kaurava and the eldest son of Dhritarashtra was always jealous of his cousins Pandava, sons of Pandu. When the eldest Pandava, Yudhishthira was announced the heir to the Hastinapur throne, Duryodhana tried to burn the five brothers in a conspiracy.
Pandava spent some time in hiding and resurfaced only after their marriage to Draupadi, the princess of Panchal kingdom. To avoid any future conflicts between his sons and nephews, Dhritarashtra decided to offer a separate part of land to Pandava, where they built the kingdom of Indraprastha. Pandava brothers went on conquest in all directions and brought all the kingdoms of Bharatavarsha under the suzerainty of Indraprastha. When Yudhishthira conducted a Rajasuya yagya, Duryodhana became extremely jealous looking at the prosperity of his cousins.
Duryodhan then conspired along with his uncle Shakuni and friend Karna to set-up a game of dice where he humiliated Pandava and their wife Draupadi in open court. Later another game of dice was played and because of losing in that game Pandava and Draupadi had to spend 13 years in exile.
After completing their 13 years of exile Pandava demanded their kingdom back from Duryodhan, which he refused. This resulted into both sets of cousins gather their allies and assemble at the battlefield of Kurukshetra to decide the fate of Bharatavarsha.
Kingdoms from all over the world picked their sides and participated in this epic battle.
In the end millions died and only a handful survived.