The answer given by Shri Krishna is equally unexpected.  He says “your present reluctance to fight is illusion. Your problem is not  regarding the fight as such but the fight against what you call my relatives,  my brothers, my friends”. Krishna says that “your real fight has to be against  ‘I' and 'My’ rather than the fight outside”. It is in this context of how to  come out of our ego i.e. ‘ I ‘ and the result of the ego ‘My' that all the  other seventeen chapters have been strung into one garland..

In teaching Arjuna, Krishna employs two sets of values,  the relative and the absolute. He begins by dealing with Arjuna’s feelings of  revulsion, on general grounds. Arjuna shrinks from the act of killing. Krishna  reminds him that, in the absolute sense, there is no such act called killing.  The Atman, the indwelling Godhead (soul) is the only reality. This body is  simply an appearance; its existence, its destruction, is likewise, illusory.

Having said this, Krishna goes on to discuss Arjuna’s  individual problem. For Arjuna, a member of the warrior caste, the fighting of  this battle is undoubtedly ‘righteous’. His cause is just. To defend it is his  duty. Running away from the battle is avoiding duty and escapism.

Socially the caste system is graded, but spiritually,  there are no such distinctions. Everyone, says Krishna, can attain the highest  sainthood by following the prescribed path of his own caste duty. There have  been instances of men everywhere who grew into spiritual giants while carrying  out their duties as merchants, peasants, doctors, priests, or kings.

In the purely physical sphere of action, Arjuna is,  indeed, no longer a free agent. The act of war is upon him; it has evolved out  of his previous actions. It is his svadharma. At any given moment in  time, we are what we are; and we have to accept the consequences of being  ourselves. Only through this acceptance can we begin to evolve further. We may  select the battleground. We cannot avoid the battle.

Arjuna is bound to act, but he is still free to make his  choice between two different ways of performing that action. In general,  mankind almost always acts with attachment; that is to say, with desire and  fear. Desire for a certain result and fear that this result will not be  obtained. Actions with attachments bind us to the world of appearances; to the  continual doing of more actions.

But there is another way of performing action, and this  is without desire and without fear. The doer of the non-attached actions is the  most conscientious of men. Freed from desire and fear, he offers everything he  does as a sacrament of devotion to his duty (surrenders all his actions to the  Lord). All work becomes equally and vitally important. It is only toward the  results of work- success or failure, praise or blame- that he remains  indifferent. When action is done in this spirit, Krishna teaches, it will lead  to the knowledge of what is behind action, behind all life; the ultimate  Reality. And, with the growth of this knowledge, the need for further action  will gradually fall away from us. We shall realize our true nature, which is God, sat-chit-ananda.
  It follows, therefore, that every action, under certain  circumstances and for certain people, may be a stepping-stone to spiritual  growth – if it is done in the spirit of non-attachment. All good and all evil  is relative to the individual point of growth. For each individual, certain  acts are absolutely wrong. Indeed, there may well be acts that are absolutely  wrong for every individual alive on earth today. But, in the highest sense,  there can be neither good nor evil. Krishna, therefore speaking as God Himself,  advises Arjuna to fight. The Gita thus neither sanctions war nor condemns it.  Regarding no action as of absolute value, either for good or for evil, it  cannot possibly do either. (Swami Prabhavananda).

Dharma and satya were at stake in Kurukshetra. So,  preventing adharma from gaining victory over dharma was the  purpose of Mahabharata war and fighting for dharma against adharma is the message of Gita.

However, we have forgotten this message of Gita and have  distorted it in the name of ahimsa as our dharma unconditionally.  Our dharma was satya (truth), and our duty was to fight and protect dharma and  satya from every enemy. Dharmao rakshati rakshitah - dharma protects  those who protect it - is our creed. And violence was not prohibited in this  fight for satya and dharma. Otherwise Rama would not have killed Vali or  Ravana. Actually, violence committed for ensuring dharma by a kshatriya is no  violence. That is why Krishna asks Arjuna  in each and every chapter of the Gita “Arise Arjuna, pick up your weapon and fight to defeat  adharma”. So, we will have to hear the teachings of Krishna if we want to  prevent the down sliding of the humanity.

To sum up, war is justified only when it is meant to  fight evil and injustice and not for the purpose of self aggrandizement.

2.  How such a long discourse like the Gita took place in the midst of two  impatient armies ready to fight it out?

The  rules of time and space as we understand them today were not applicable to the  age when the Mahabharata war took place during which the discourse was delivered  by God. What seems to us a long dialogue must have taken place in the blink of  an eye on the battlefield!  We come  across many stories indicating that silence is more powerful and penetrating  than speech and a teacher taught his students by maintaining silence - thought  transference or telepathy. These instances might be a pre-cursor to the modern developments  in the field of information technology.