The asuras want to enjoy a life of sense gratification, even at the cost of others' happiness. In order to fulfill this ambition, the asuras, especially atheistic kings or state executive heads, try to equip themselves with all kinds of deadly weapons to bring about a war in a peaceful society. They have no ambition other than personal aggrandizement, and thus mother earth feels overburdened by such undue increases of military strength. By increase of the asuric population, those who follow the principles of religion become unhappy, especially the devotees, or devas.
In such a situation, the Personality of Godhead incarnates to vanquish the unwanted asuras and to reestablish the true principles of religion. This was the mission of Lord Śrī Kṛṣṇa, and He fulfilled it.