This question has hounded every devout Buddhist since the days of the Buddha. Happily, two renderings from Him on the subject are available for perusal. Unhappily, though, the translators forgot to say where the words were quoted from – Editor

You can be a soldier of Truth, but not the aggressor.

One day, Sinha, the general of the army, went to the Buddha and said, ‘I am a soldier, O Blessed One. I am appointed by the King to enforce his laws and to wage his wars. The Buddha teaches infinite love, kindness and compassion for all sufferers: Does the Buddha permit the punishment of the criminal? And also, does the Buddha declare that it is wrong to go to war for the protection of our homes, our wives, our children and our property? Does the Buddha teach the doctrine of complete self-surrender? Should I suffer the evildoer to do what he pleases and yield submissively to him who threatens to take by violence what is my own? Does the Buddha maintain that all strife including warfare waged for a righteous cause should be forbidden?’

The Buddha replied, ‘He who deserves punishment must be punished. And he who is worthy of favor must be favored. Do not do injury to any living being but be just, filled with love and kindness.’ These injunctions are not contradictory because the person who is punished for his crimes will suffer his injury not through the ill-will of the judge but through the evil act itself. His own acts have brought upon him the injury that the executors of the law inflict. When a magistrate punishes, he must not harbor hatred in his heart. When a murderer is put to death, he should realize that his punishment is the result of his own act.
With his understanding, he will no longer lament his fate but can console his mind. And the Blessed One continued, ‘The Buddha teaches that all warfare in which man tries to slay his brothers is lamentable. But he does not teach that those who are involved in war to maintain peace and order, after having exhausted all means to avoid conflict, are blameworthy.

‘Struggle must exist, for all life is a struggle of some kind. But make certain that you do not struggle in the interest of self against truth and justice. He who struggles out of self-interest to make himself great or powerful or rich or famous, will have no reward. But he who struggles for peace and truth will have great reward; even his defeat will be deemed a victory.

‘If a person goes to battle even for a righteous cause, then, Sinha, he must be prepared to be slain by his enemies because death is the destiny of warriors. And should his fate overtake him, he has no reason to complain. But if he is victorious his success may be deemed great, but no matter how great it is, the wheel of fortune may turn again and bring his life down into the dust. (This one does not appear in the second version) However, if he moderates himself and extinguishes all hatred in his heart, if he lifts his down-trodden adversary up and says to him, ‘Come now and make peace and let us be brothers,’ then he will gain a victory that is not a transient success; for the fruits of that victory will remain forever.

‘Great is a successful general, Sinha, but he who conquers self is the greater victor. This teaching of conquest of self, Sinha, is not taught to destroy the lives of others, but to protect them. The person who has conquered himself is more fit to live, to be successful and to gain victories than is the person who is the slave of self. The person whose mind is free from the illusion of self, will stand and not fall in the battle of life. He whose intentions are righteousness and justice, will meet with no failures. He will be successful in his enterprise and his success will endure. He who harbors love of truth in his heart will live and not suffer, for he has drunk the water of immortality. So struggle courageously and wisely. Then you can be a soldier of truth.’

Excerpt, What Buddhists believe, by Ven. K. Sri Dhammananda, 1993

The following is a different translation from “Sayings of Buddha”, Pilgrims Publishing, 2003.

Simha said: “One doubt still lurks in my mind concerning the doctrine of the Blessed One. I am a soldier, and am appointed by the king to enforce his laws and to wage his wars. Does the Tathagata declare that it is wrong to go war for the protection of our homes, our wives, our children, and our property? Does the Tathagata teach the doctrine of a complete self surrender? Does the Tathagata maintain that warfare waged for a righteous cause should be forbidden? Buddha replied:

He who deserves punishment must be punished, and he who isworthy of favor must be favored. Yet at the same time the Tathagata teaches to do no injury to any living being but to be full of love and kindness. These injunctions are not contradictory, for whosoever must be punished for the crimes, which he has committed, suffers his injury not through the ill-will of the judge put on account of his evil-doing. His own acts have brought upon him the injury that the executor of the law inflicts. When a magistrate punishes, let him not harbor hatred in his breast; and a murderer, when put to death, should consider that this is the fruit of his own act.

The Tathagata teaches that all warfare in which man tries to slay his brother is lamentable, but he does not teach that those who go to war in a righteous cause, after having exhausted all means to preserve the peace, are blameworthy. He must be blamed who is the cause of war. The Tathagata teaches a complete surrender of self, but he does not teach a surrender of anything to those powers that are evil, be they men or gods or the elements of nature.

Struggle must be, for all life is a struggle of some kind. But he that struggles should look to it lest he struggle in the interest of self against truth and righteousness. He who struggles in the interest of self, so that he himself may be great or powerful or rich or famous, will have no reward, but he who struggles for righteousness and truth, will have great reward, for even his defeat will be a victory.

Self is not a fit vessel to receive any great success; self is small however, is large enough to receive the yearning and aspirations of all selves and when the selves break like soap bubbles, their contents will be preserved and in the truth they will lead a life everlasting. (This does not appear in the first version)

He who goes to battle, O Simha, even though it be in a righteous cause, must be prepared to be slain by his enemies, for that is the destiny of warriors; and should his fate overtake him he has no reason for complaint. But he who is victorious should remember the instability of earthly things. His success may be great, but be it ever so great the wheel of fortune may turn again and bring him down into the dust.

The doctrine of the conquest of self, O Simha, in not taught to destroy the souls of men, but to preserve them. He who has conquered self is more fit to live, to be successful, and to gain victories that he who is the slave of self. He, whose mind is free from the illusion of self, will stand and not fall in that battle of life. He, whose intention are righteousness and justice, will meet with no failure, but be successful in his enterprises and his success will endure. He who harbors in his heart love of truth will live and not die, for he has drunk the water of immortality. Struggle then, O general, courageously; and fight your battles vigorously, but be a soldier of truth, and the Tathagata will bless you.