The Gita is set on a battlefield. Two armies stand ready, and the Lord tells the warrior to rise and fight. He says that the enemies are already slain by him and that Arjuna is to be merely an instrument. A reader can fairly ask: is this book teaching war and violence?
The answer is no, and the reason lies in where the call is made, to whom, and under what measure. A single situation is not a rule for everyone.
Why people read it this way
The first scene is a war. In the second chapter the Lord tells Arjuna to rise, resolved to fight (Gita 2.37). In the eleventh he says that the enemies have already been slain by him, and that Arjuna is to be merely an instrument (Gita 11.33). Taken alone, these lines sound like a command for all people and all times. That is why the claim is so common.
The book begins with the refusal to fight
The Gita does not begin with a call to fight. It begins with the warrior refusing. Arjuna's body fails him. He says that he wants neither victory nor a kingdom (Gita 1.32), that he would rather be killed unresisting than kill his own people (Gita 1.46), and he throws down his bow (Gita 1.47). Lord Krishna's first response is to see him tearful and troubled (Gita 2.1). All the rest of the Gita is an answer to this refusal. The full account of the scene is in the detailed reading.
The call is made to one man, for one duty
तस्मादुत्तिष्ठ कौन्तेय युद्धाय कृतनिश्चयः ॥ २.३७ ॥
tasmād uttiṣhṭha kaunteya yuddhāya kṛitaniśhchayaḥ || 2.37 ||
Either, being slain, you will attain heaven, or, being victorious, you will enjoy the earth. Therefore, arise, O son of Kunti, resolved to fight.
The Lord appeals to Arjuna's own duty as a warrior, to his honour, and to what will follow (Gita 2.31–37). He speaks of a war that "comes unsought" (Gita 2.32) and calls it a righteous battle (Gita 2.33). Every line is addressed to a particular man in a particular hour. No line says that anyone else should take up arms on the same ground. The seven verses are read one by one in Duty, Honour and Fear.
The measure the Lord puts on the act
The call to fight comes with a condition. Arjuna is to treat pleasure and pain, gain and loss, victory and defeat alike, and then engage in battle (Gita 2.38). Later he is told to act with his mind fixed on the Self, free from expectation and ownership, and free from mental fever (Gita 3.30). The act is asked for. The heat, the hatred and the hunger for the result are not.
What the Gita praises in a person
The Gita's own lists of excellence put non-violence first. Among the divine qualities are non-violence, truthfulness, freedom from anger and compassion toward all beings (Gita 16.2). The devotee the Lord holds dear bears hatred toward no being and is friendly and compassionate to all (Gita 12.13). Non-violence is named among the qualities that arise from him (Gita 10.5).
दया भूतेष्वलोलुप्त्वं मार्दवं ह्रीरचापलम् ॥ १६.२ ॥
dayā bhūteṣhv aloluptvaṁ mārdavaṁ hrīr achāpalam || 16.2 ||
Non-violence, truthfulness, freedom from anger, renunciation, tranquility, aversion to faultfinding, compassion towards all living beings, freedom from covetousness, gentleness, modesty and steadiness are further qualities of the divine nature.
A book that held violence as its teaching would not set these qualities at the head of its list.
What "merely an instrument" means
The line about the instrument is spoken after Arjuna has seen the whole and has asked to be taught. The Lord says that the enemies are already slain by him, and that Arjuna is to be merely the instrument (Gita 11.33). It is said to a man who has surrendered the sense of being the doer. It is not a licence for anyone to say that a higher will acts through them. The verse is read in full in Merely an Instrument.
How the epic itself treats the war
The Gita is a chapter of the Mahabharata, and the epic does not celebrate the war it describes. After the fighting, the epic turns to the women of both houses and to Gandhari's grief for her sons. The victor Yudhishthira is so shaken by the slaughter of his kin that he wants to give up the throne. A reader who takes the Gita as a call to war has to ignore the book it sits in, which ends in mourning and not in triumph.
The better reading
The Gita speaks to a person who refused his duty out of confusion, and it gives him an answer for that situation, with a measure of equal mind, no hatred and no claim to the result. It does not hand out a general call to violence, and its own picture of the good person is the opposite of a hater.
Where to read next
To see why Arjuna refused, and what the Lord first says to him, read the detailed reading of this hub. The teaching that comes before the call to duty is in The Imperishable Self.