Link to purport by A C Bhaktivedanta Swami Srila Prabhupada

Transcript of Bhakti-Shastri class on this verse by Chaitanya Charan

Bg 18.59

In previous verse

atha cet tvam ahaṅkārān
na śroṣyasi vinaṅkṣyasi

 so now Krishna will tell how when ahankara is there vinaṅkṣyasi takes place.

yad ahaṅkāram āśritya
na yotsya iti manyase
mithyaiṣa vyavasāyas te
prakṛtis tvāṁ niyokṣyati

Synonyms: 

yat — if; ahaṅkāram — of false ego; āśritya — taking shelter; na yotsye — I shall not fight; iti — thus; manyase— you think; mithyā eṣaḥ — this is all false; vyavasāyaḥ — determination; te — your; prakṛtiḥ — material nature; tvām — you; niyokṣyati — will engage.

Translation: 

If you do not act according to My direction and do not fight, then you will be falsely directed. By your nature, you will have to be engaged in warfare.

So instead of taking shelter of Me if you take shelter of Ahankara, that I am powerful, I am strong, I know what to do and I will do myself 

yad ahaṅkāram āśritya, 

na yotsya iti manyase

if you think that I will not fight na yotsya, 

mithyaiṣa vyavasāyas te, mithyaiṣa, this is all false,

vyavasāyas te, so vyavasā means determination , determination will come falsely directed 

and prakṛtis tvāṁ niyokṣyati, material nature will engage you,

So Srila Prabhupada has written that Arjuna is a Kshetriya so he will fight whether by Krishna’s will or by his independent nature.

End of transcription.