In plain words
Wherever there is Krishna, the Lord of Yoga, and wherever there is Partha, the wielder of the bow, there are prosperity, victory, flourishing, and enduring nīti—this is my conviction.
Word for word
Where Kṛṣṇa, the Lord of yoga, is, where Pārtha, the bearer of the bow, is, there are fortune, victory, prosperity, unfailing right conduct: [this is] my conviction.
The commentators
Bhagavadgītābhāṣyam
किं बहुना —
यत्र यस्मिन् पक्षे योगेश्वरः सर्वयोगानाम् ईश्वरः, तत्प्रभवत्वात् सर्वयोगबीजस्य, कृष्णः, यत्र पार्थः यस्मिन् पक्षे धनुर्धरः गाण्डीवधन्वा, तत्र श्रीः तस्मिन् पाण्डवानां पक्षे श्रीः विजयः, तत्रैव भूतिः श्रियो विशेषः विस्तारः भूतिः, ध्रुवा अव्यभिचारिणी नीतिः नयः, इत्येवं मतिः मम इति ॥ ७८ ॥
In plain words
Why say more? —
Yatra — meaning: on whichever side stands the Lord of yoga, master of every yoga, since every yoga rises out of him who is its seed; Kṛṣṇa. And where Pārtha stands, bow in hand, the Gāṇḍīva his bow. There is śrī: on that side, the Pāṇḍavas' side, śrī means victory. And there too is bhūti: bhūti is that same prosperity carried further, spread wide. And nīti — sound strategy — dhruvā, never failing there. Just this is my conviction.
Close to the Sanskrit
What [is gained] by much [speaking]? —
Yatra — on whichever side — yogeśvaraḥ, the Lord of all yogas, because they arise from him who is the seed of all yoga; Kṛṣṇa; yatra — on whichever side — Pārtha, dhanurdharaḥ, the bearer of the bow, he whose bow is the Gāṇḍīva; tatra śrīḥ — on that side, the side of the Pāṇḍavas, śrī, that is, victory; there itself bhūtiḥ — bhūti is the further degree, the wide expansion, of that śrī; dhruvā, unfailing, nītiḥ, that is, sound policy: thus, in this way, is the conviction — mine. Thus.