SB 1.16.34

yo vai mamatibharam asura-vamsa-rajñam
aksauhini-satam apanudad atma-tantrah
tvam duhstham una-padam atmani paurusena
sampadayan yadusu ramyam abibhrad angam

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:

O personality of religion, I was greatly overburdened by the undue military phalanxes arranged by atheistic kings, and I was relieved by the grace of the Personality of Godhead. Similarly you were also in a distressed condition, weakened in your standing strength, and thus He also incarnated by His internal energy in the family of the Yadus to relieve you.

Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:

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 Sri Krsna, and He fulfilled it.