Visiting without being asked, not running away from one's proper duties.
Krishna describes this battle as "a gateway to heaven." However, he is not praising aggression. The Pandavas sought peace and only desired the territories that rightfully belonged to them. Nevertheless, they could not avoid conflict, and a duty to uphold justice was placed before them.
What is emphasized here is not a battle that one initiates in pursuit of personal gain or fame, but rather the righteous duty that comes unbidden. Because fulfilling this obligation without evading it leads to spiritual growth, for Kshatriyas, this opportunity is called "an open gate."
Japanese Translation from the Geeta Press Edition - Chapter 2, Section 32: "The Righteous Battle as an Open Gate to Heaven"
Sanskrit original text
यदृच्छया चोपपन्नं स्वर्गद्वारमपावृतम् ।
सुखिनः क्षत्रियाः पार्थ लभन्ते युद्धमीदृशम् ॥ ३२ ॥Romanization
yadṛcchayā copapannaṃ svargadvāram apāvṛtam |
sukhinaḥ kṣatriyāḥ pārtha labhante yuddham īdṛśam || 32 ||Arjuna, a Kshatriya who has such an opportunity to engage in battle that comes without being asked is fortunate, for it is like a gateway to heaven. (32)
In this section, the speaker is calling Arjuna "Pārtha." By doing so, the speaker is reminding us of Pṛthā, which is also the name of Arjuna's mother. This reminder refers to a message that the speaker brought with him from Hastinapura.
The message was as follows:
"Please tell Arjuna and Bima that they must always be ready for battle. The time has come that a Kshatriya mother waits for when her son is born."
By referring to this battle as an "opportunity that came without being sought," the Lord is trying to make Arjuna understand that it was not a crisis they faced by their own desires.
The Pandavas had exhausted every possible means to bring about a friendly resolution. However, Duryodhana refused the proposal to return the kingdom of the Pandavas, which was under his management in trust, without resorting to war as a judgment.
He clearly declared that he would not give the Pandavas even a piece of land that could be covered by the tip of a needle. At that time, the Pandavas were forced to prepare for war. Therefore, it was an "uninvited battle" for the Pandavas.
By declaring this battle "the open gate to heaven," it is shown that warriors who meet their end in such a righteous fight can directly enter the heavens, and that nothing will obstruct their path.
"Such an opportunity is a blessing for a Kshatriya," the sentence indicates that not all Kshatriyas encounter such righteous battles.
Only the most fortunate among the Kshatriyas have the privilege of facing such battles as their duty.
For Arjuna, it was a great fortune to have the opportunity to participate in such a righteous battle without seeking it himself. Therefore, he should not consider turning away from it under any circumstances.
In the footnotes, references are made to Mahābhārata, Udyoga Parva, 137.9 through 10.