It is a well-known fact that Krishna is also known as "Ranchor", a name he earned after running away from a battle with Jarasandha.
But, how is it that the same Jarasandha was defeated by Karna?
So, what does Krishna running way imply?
Was Krishna incompetent against Jarasandha, or something else?
No, Krishna was not incompetent against Jarasandha. In fact, Krishna and his army has defeated Jarasandha and his army eighteen times.
Krishna running away from Jarasandha implies that, Jarasandha was destined to be killed by Bhima in duet.
This story is mentioned in Harivamsa Purana (Khila of Mahabharata) as follows:
Realising that jarAsaMdha was skilled in mace fight, valiant balarAma became angry. He abandoned the mace and picked up his best club.
Seeing the angry balarAma picking up the infallible club, terrible at sight, in the great battle,
the voice of the witness of the world, was heard in the sky, telling baladeva (balarAma), who had picked up his weapon, the club.
O rAma (balarAma)! mAgadha (jarAsaMdha) is not to be killed by you. You need not lament. I have foreseen his death. Hence control yourself.
In a short time, the lord of magadha (jarAsaMdha) will lose his life.
Hearing this, jarAsaMdha became disappointed as if he lost his mind. balarAma, having club as his weapon, did not strike jarAsaMdha again.
Both stopped fighting. The vR^iShNi-s and the kings also stopped fighting. In this way the terrible battle between them became devoid of any interest.
(vaishampAyana said) O great king (janamejaya)! (In this way, the terrible battle that resulted in ) killing each other for a long time came to an end.
When jarAsaMdha was defeated, he left the battlefield. As the sun set, the yAdava-s did not chase him in the night.
Assembling their forces, the (yAdava-s) warriors of great power, protected by keshava (kR^iShNa), achieving their aim, happily entered the city.
king jarAsaMdha, disappointed, returned to his city. The kings who followed him also returned to their kingdoms.
O tiger of the kuru race (janamejaya)! Even though they defeated jarAsaMdha, the vR^iShNI-s did not think so. The king jarAsaMdha is most powerful.
The yAdava-s fought with jarAsaMdha ten and eight (eighteen) battles. The powerful yAdava-s were unable to kill him.
O One with great intellect (janamejaya)! king jarAsaMdha has twenty akShauhiNi-s who came for fighting the battle for him.
O the bull of bharata race! Compared to jarAsaMdha's army, the army of vR^iShNi-s is meager. O the king of kings (janamejaya)! they were baffled on being attacked by jarAsaMdha along with other kings.
jarAsaMdha again came prepared and attacked yAdavas, protected by kR^iShNa.
After defeating jarAsaMdha, the king of magadha in the battle, the lions of the vR^iShNi race, the great chariot fighters lived comfortably.
"...from fear of Jarāsandha, at one time, we had to leave Mathurā", says Kṛṣṇa in Sabhā Parva of the Mahābhārata. I don't think it signifies anything else.
Kṛṣṇa explains to Yudhiṣṭhira as follows.
Then, O great king, remembering the conclusion to which we had come of old we became exceedingly cheerless and fled from Mathura. Dividing our large wealth into small portions so as to make each portion easily portable, we fled from fear of Jarasandha, with our cousins and relatives. Reflecting upon everything, we fled towards the west. There is a delightful town towards the west called Kusasthali, adorned by the mountains of Raivata. In that city, O monarch, we took up our abode. We rebuilt its fort and made it so strong that it has become impregnable even to the Gods. And from within it even the women might fight the foe, what to speak of the Yadava heroes without fear of any kind? O slayer of all foes, we are now living in that city. And, O tiger of the Kuru race, considering the inaccessibility of that first of mountains and regarding themselves as having already crossed the fear of Jarasandha, the descendants of Madhu have become exceedingly glad. Thus, O king, though possessed of strength and energy, yet from the oppressions of Jarasandha we have been obliged to repair to the mountains of Gomanta, measuring three Yojanas in length. Within each yojana have been established one and twenty posts of armed men. And at intervals of each yojana are hundred gates with arches which are defended by valourous heroes engaged in guarding them. And innumerable Kshatriyas invincible in war, belonging to the eighteen younger branches of the Yadavas, are employed in defending these works. In our race, O king, there are full eighteen thousand brothers and cousins.
...
By defeating the kings with their troops and bringing all of them as captives into this city, he had swelled its crowds enormously. We also, O king, from fear of Jarāsandha, at one time had to leave Mathurā and fly to the city of Dwaravati.
The same is also explained in this YouTube video from the 1988 Mahabharat TV series by B. R. Chopra.
O rAma (balarAma)! mAgadha (jarAsaMdha) is not to be killed by you. You need not lament. I have foreseen his death. Hence control yourself.
Commented
Dec 9, 2016 at 3:50
As told by another user in his answer, Incompetence is not at all the reason for Krishna.
There are two key reasons behind this
Krishna did not choose to kill Jarasandha to make his task easier and
Krishna obeyed the command of Brahma that Jarasandha has to be Killed by Bhima only
Now, I am providing the scriptural statements for both reasons from the Mahabharata and Śrīmad-Bhāgavatam.
It is well-known that the intention behind Krishna avatar is to ease the earth by removing her burden (i.e., danavas, asuras). It can be clarified from the following statements of Mahabharata
"Vaisampayana continued, 'Having said so unto the Earth, O king, the divine Brahman bade her farewell. And the Creator then commanded all the gods saying, 'To ease the Earth of her burden, go ye and have your births in her according to your respective parts and seek ye strife (with the Asuras already born there)'. And the Creator of all, summoning also all the tribes of the Gandharvas and the Apsaras, spake unto them these words of deep import, 'Go ye and be born amongst men according to your respective parts in forms that ye like.'
"And all the gods with Indra, on hearing these words of the Lord of the celestials--words that were true, desirable under the circumstances, and fraught with benefit,--accepted them. And they all having resolved to come down on earth in their respected parts, then went to Narayana, the slayer of all foes, at Vaikunth--the one who has the discus and the mace in his hands, who is clad in purple, who is of great splendour, who hath the lotus on his navel, who is the slayer of the foes of the gods, who is of eyes looking down upon his wide chest (in yoga attitude), who is the lord of the Prajapati himself, the sovereign of all the gods, of mighty strength, who hath the mark of the auspicious whirl on his breast, who is the mover of every one's faculties and who is adored by all the gods. Him, Indra the most exalted of persons, addressed, saying, "Be incarnate." And Hari replied,--'Let it be.'"
[Section 64, Adivansavatarana Parva, Adi Parva, The Mahabharata]
In order to accomplish his purpose, Krishna use to leave Jarasandha after destroying all the army of Jarasandha. You can notice that Krishna stopped Balarama from killing Jarasandha in the same chapter of Śrīmad-Bhāgavatam.
Although Lord Kṛṣṇa, the Supreme Personality of Godhead, is the original cause of this world, when He descended to the earth He played the role of a human being. Thus when He saw Jarāsandha’s assembled army surrounding His city like a great ocean overflowing its shores, and when He saw how this army was striking fear into His subjects, the Lord considered what His suitable response should be according to the time, place and specific purpose of His current incarnation. [The Supreme Lord thought:] Since it is such a burden on the earth, I will destroy Jarāsandha’s army, consisting of akṣauhiṇīs of foot soldiers, horses, chariots and elephants, which the King of Magadha has assembled from all subservient kings and brought together here. But Jarāsandha himself should not be killed, since in the future he will certainly assemble another army. This is the purpose of My present incarnation — to relieve the earth of its burden, protect the pious and kill the impious.
[5-9, Chapter 50: Kṛṣṇa Establishes the City of Dvārakā, Canto 10, Śrīmad-Bhāgavatam]
Krishna implemented the same policy for seventeen times i.e., Krishna use to destroy the total army of Jarasandha and then leaving Jarasandha alone.
Seventeen times the King of Magadha met defeat in this very way. And yet throughout these defeats he fought on with his akṣauhiṇī divisions against the forces of the Yadu dynasty who were protected by Śrī Kṛṣṇa. By the power of Lord Kṛṣṇa, the Vṛṣṇis would invariably annihilate all of Jarāsandha’s forces, and when all his soldiers had been killed, the King, released by his enemies, would again go away.
[41-42, Chapter 50: Kṛṣṇa Establishes the City of Dvārakā, Canto 10, Śrīmad-Bhāgavatam]
But Krishna decided to shift Yadus, the eighteenth time, for the benefit of inhabitants only.
Just as the eighteenth battle was about to take place, a barbarian warrior named Kālayavana, sent by Nārada, appeared on the battlefield. Arriving at Mathurā, this Yavana laid siege to the city with thirty million barbarian soldiers. He had never found a human rival worth fighting, but he had heard that the Vṛṣṇis were his equals. When Lord Kṛṣṇa and Lord Saṅkarṣaṇa saw Kālayavana, Kṛṣṇa thought about the situation and said, “Ah, a great danger now threatens the Yadus from two sides. This Yavana is besieging us already, and the mighty King of Magadha will soon arrive here, if not today then tomorrow or the next day. If powerful Jarāsandha comes while We two are busy fighting Kālayavana, Jarāsandha may kill Our relatives or else take them away to his capital. Therefore We will immediately construct a fortress that no human force can penetrate. Let Us settle our family members there and then kill the barbarian king.”
[43-48, Chapter 50: Kṛṣṇa Establishes the City of Dvārakā, Canto 10, Śrīmad-Bhāgavatam]
Thus, Krishna did not runaway from any war. He took safety measure for Yadus and then killed Kālayavana. It was the decision by Krishna due to the participation of Kalayavana.
It is the command of Brahma that Jarasandha is not destined to die in the hands of Bhima only. Krishna obeyed the command of Brahma and hence refrained himself in doing so. It can be clarified from the following statements of Mahabharata
Vaisampayana said,--"Having spoken thus, and desiring to fight with those heroes of terrible achievements, king Jarasandha ordered (his son) Sahadeva to be installed on the throne. Then, O bull of the Bharata race, the king, on the eve of battle, thought of his two generals Kausika and Chitrasena. These two, O king, were formerly called by everybody in the world of men by the respectful appellations of Hansa and Dimvaka. And, O monarch, that tiger among men, the lord Sauri ever devoted to truth, the slayer of Madhu, the younger brother of Haladhara, the foremost of all persons having their senses under complete control, keeping in view the command of Brahma and remembering that the ruler of Magadha was destined to be slain in battle by Bhima and not by the descendant of Madhu (Yadavas), desired not to slay himself king Jarasandha, that foremost of all men endued with strength, that hero possessed of the prowess of a tiger, that warrior of terrible valour."
[Section 22, Jarasandhta-badha Parva, Sabha Parva, The Mahabharata]