Srimad Bhagvata Mahapurana: Book 10: Chapter 50: Verses 1-16
The Lord ensconces Himself in a fortress (built by Him in the western sea under the name of Dwaraka)
Sri Suka began again : Their husband being dead, Asti and Prapti, the two wives of Kamsa, 0 Pariksit (the foremost of the Bharatas), went away stricken with sorrow to the house of their father: so the tradition goes. Seized with agony they made known to their father, Jarasandha, the ruler of the Magadhas, all that had contributed to their widowhood. Full of grief and indignation of hear that unpleasant story, 0 protector of men, Jarasandha put forth a supreme endeavour to rid the earth of the Yadavas. Protected by three and twenty Aksauhinis he besieged Mathura, the capital of the Yadus, on all sides. Taking a full view of his army, which resembled an ocean that had overflowed its banks, of His own city besieged by it and of His people (near and dear ones) stricken with fear, Sri
Krsna--who was no other than Lord Sri Hari (Visnu) appearing in a human semblance from some motive (viz., for removing the burden of the earth), pondered over (reviewed) the purpose of His descent (on this earthly plane) as well as on what was equal to the time and place in question.
(He took no time in arriving at a decision and said to Himself,) "I shall certainly destroy this army of all dependent chiefs, brought together by Jarasandha (the ruler of the Magadhas), reckoned as many Aksauhinis strong and consisting of foot soldiers, cavalry, chariots and elephants--a concentrated burden on the earth (and therefore easy to dispose of). Jarasandha, however, is not to be killed (at once) as (in the event of his being let off) he is expected to put forth renewed efforts to mobilize an army (so that I shall be spared the trouble of hunting up evil-doers). This present descent of Mine is directed towards this very end, viz., removing the earth's burden, protecting the virtuous and wiping out the vile. (why this alone,) other (divine) forms too (such as those of a boar and a man-lion) are duly assumed by Me (from time to time) for the vindication of righteousness as well as for putting a stop to (the mad career of) unrighteousness gaining ascendancy during certain periods." While Sri Krsna (the Protector of cows) was thus contemplating, there presently alighted from the heavens two chariots, possessing the effulgence of the sun and (each) accompanied by a charioteer and fitted with (necessary) appurtenances (such as armour and ensign).
There also appeared of their own accord eternal divine weapons (of the Lord). Perceiving them, Sri Krsna (the Controller of the senses) immediately spoke to Sankarsana (as follows):- "See, noble Brother, the calamity that has come upon the Yadus, of whom You are the Guardian, 0 Lord ! Here is Your chariot come, as well as Your favourite weapons (the plough, pestle and soon), Mounting the vehicle, sweep off this army and completely deliver Your people (thereby) from the disaster (that stares them in the face). Surely to this end is directed Our descent, which brings happiness to the virtuous, 0 Lord ! Remove the earth's burden going by the name of the twenty-three Aksauhinis." Having thus fully deliberated, Balarama and Sri Krsna (the two Scions of Dasarha) rode out of the city in their respective chariots, clad in armour and armed with Their own weapons and surrounded by a very small detachment. Coming out of the city into the open land, Sri Hari, who had Daruka for His charioteer, blew His conch (thereby challenging the enemy to encounter Them).