SB 10.50.5-6

nirīkṣya tad-balaṁ kṛṣṇa
 udvelam iva sāgaram
sva-puraṁ tena saṁruddhaṁ
 sva-janaṁ ca bhayākulam
cintayām āsa bhagavān
 hariḥ kāraṇa-mānuṣaḥ
tad-deśa-kālānuguṇaṁ
 svāvatāra-prayojanam
Word for word: 
nirīkṣya — observing; tat — of him (Jarāsandha); balam — the military force; kṛṣṇaḥ — Lord Kṛṣṇa; udvelam — having overflowed its boundaries; iva — like; sāgaram — an ocean; sva — His own; puram — city, Mathurā; tena — by it; saṁruddham — besieged; sva-janam — His subjects; ca — and; bhaya — by fear; ākulam — disturbed; cintayām āsa — He thought; bhagavān — the Supreme Personality of Godhead; hariḥ — Lord Hari; kāraṇa — the cause of everything; mānuṣaḥ — appearing as a human being; tat — for that; deśa — place; kāla — and time; anuguṇam — suitable; sva-avatāra — of His descent to this world; prayojanam — the purpose.
Translation: 
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.
Purport: 

The ācāryas point out that the Supreme Godhead did not have to worry about a mortal attack from Jarāsandha and his soldiers. But, as stated here, Śrī Kṛṣṇa was playing the part of a human being (kāraṇa-mānuṣaḥ), and He played the part well. This play is called līlā, the Lord’s enactment of spiritual pastimes for the pleasure of His devotees. Although ordinary persons may be dumbfounded by the Lord’s pastimes, the devotees derive tremendous pleasure from His inimitable style of behavior. Thus Śrīla Śrīdhara Svāmī points out that Śrī Kṛṣṇa thought as follows: “How should I defeat Jarāsandha? Should I kill the army but not Jarāsandha, or should I kill Jarāsandha and take the army for Myself? Or perhaps I should just kill both of them.” Lord Kṛṣṇa’s conclusion is described in the following verses.