We can scarcely imagine the difficulty faced by Arjuna on the battlefield of Kurukshetra. Although the enemy party consisted of those who were truly the enemies of all humankind, some of their number consisted of people whom Arjuna had known throughout his life - cousins, classmates, and his military teacher Drona among them (Bg. 1.26). They had, alas, chosen the wrong side; they had chosen to fight for Duryodhana.
Krishna, as Arjuna's cousin and lifelong friend, and as the Supreme Personality of Godhead, understood that for Arjuna to do what he was sworn to do as a Ksatriya - protect humanity from criminal abusers opposed to the path of dharma - would be a nearly insurmountable challenge for him given the makeup of the opposing party. Fortunately for Arjuna, he had selected Krishna for his charioteer, and so would get all the help he would need, both in terms of moral support and practical guidance.
On the battlefield of Kurukshetra, Krishna exhibited His opulence of renunciation. As Sri Krishna is the Godhead Himself, it was possible for Him to consistently insist that Arjuna do what was right - to fight and kill any and all members of the opposing party, despite their previously close relationships. This is an issue that dogs many commentators on the Bhagavad-gita: that the supreme arbiter of justice and morality should instruct his disciple and friend to kill even those with whom he had been formerly friendly, and his teachers, the heretofore righteous Drona and Bhishma. But removing demoniac persons and their supporters, whose chief purpose in life is to exploit others, is one of the main reasons for Krishna's descent and appearance in human society (Bg. 4.7). And Arjuna, as his friend and confidant, would need to help in this mission, regardless of personal considerations. This is one of the main tensions and provides the chief dramatic backdrop of the Gita.
The opulence of renunciation (Bg. 18.78, purport), which allows Sri Krishna to be completely full in Himself, without the need of any other relationship, would be necessary to guide His friend Arjuna forward on the path of dharma. And thus, with the vision of eternity, Krishna was able to guide Arjuna to do his duty, despite Arjuna's seeming* personal and subjective difficulties.
* As a soul completely surrendered to Krishna, Arjuna is immune from contact with Maha Maya. He was influenced by Yoga Maya, Krishna's special energy for His divine Lila.
