यदृच्छया चोपपन्नं स्वर्गद्वारमपावृतम्। सुखिनः क्षत्रियाः पार्थ लभन्ते युद्धमीदृशम् ॥

yadṛichchhayā chopapannaṁ swarga-dvāram apāvṛitam sukhinaḥ kṣhatriyāḥ pārtha labhante yuddham īdṛiśham

Happy the Kshattriyas, O Pârtha, who obtain such a fight, offered unsought as an open door to heaven.

In plain words: Krishna says warriors are lucky when a righteous fight comes to them without their seeking it. Such a fight is like an open door to heaven.