In plain words
If, taking refuge in ahaṅkāra, you think, ‘I will not fight,’ this resolve of yours is futile; your own Prakṛti will compel you.
Word for word
If, relying on ahaṅkāra, you think 'I will not fight', this resolve of yours is vain; nature will compel you.
The commentators
Bhagavadgītābhāṣyam
इदं च त्वया न मन्तव्यम् ‘स्वतन्त्रः अहम् , किमर्थं परोक्तं करिष्यामि ? ’ इति —
यदि चेत् त्वम् अहङ्कारम् आश्रित्य न योत्स्ये इति न युद्धं करिष्यामि इति मन्यसे चिन्तयसि निश्चयं करोषि, मिथ्या एषः व्यवसायः निश्चयः ते तव ; यस्मात् प्रकृतिः क्षत्रियस्वभावः त्वां नियोक्ष्यति ॥ ५९ ॥
In plain words
And don't think like this: 'I'm my own master — why should I do what someone else tells me?'
If — taking your stand on the ego — you think, "I will not fight," meaning, I am not going to do battle: if that is what you are turning over in your mind, what you have settled on, then this resolve of yours is false. Because prakṛti — your own inborn nature, the nature of a warrior — will harness you to the fight.
Close to the Sanskrit
And this is not to be thought by you: 'I am independent; for what purpose shall I do what is spoken by another?' — thus.
If — that is, in case — you, having resorted to egoism, think 'I will not fight', that is, 'I shall not do battle' — you ponder it, you form the resolve — false is this determination, this settled conclusion, of yours, of yours; because prakṛti, the innate disposition of a kṣatriya, will yoke you to it.