He asked me what were the usual causes or motives that made one country go to war with another. I answered they were innumerable, but I should only mention a few of the chief. Sometimes the ambition of princes, who never think they have land or people enough to govern: sometimes the corruption of ministers, who engage their master in a war in order to stifle or divert the clamour of the subjects against their evil administration. Difference in opinions hath cost many millions of lives: for instance, whether flesh be bread, or bread be flesh; whether the juice of a certain berry be blood or wine; whether whistling be a vice or a virtue; whether it be better to kiss a post, or throw it into the fire; what is the best colour for a coat, whether black, white, red or grey; and whether it should be long or short, narrow or wide, dirty or clean, with many more. Neither are any wars so furious and bloody, or of so long continuance, as those occasioned by difference in opinion, especially if it be in things indifferent.
 

Sometimes the quarrel between two princes is to decide which of them shall disposses a third of his dominions, where neither of them pretend to any right. Sometimes one prince quarrelleth with another, for fear the other should quarrel with him. Sometimes a war is entered upon, because the enemy is too strong, and sometimes because he is too weak. Sometimes our neighbours want the things which we have, or have the things which we want,; and we both fight, till they take ours or give us theirs. It is a very justifiable cause of war to invade a country after the people have been wasted by famine, destroyed by pestilence, or embroiled by factions amongst themselves. It is justifiable to enter into a war against our nearest ally, when one of his towns lies convenient for us, or a territory of land, that would render our dominions round and compact. If a prince send forces into a nation where the people are poor and ignorant, he may lawfully put half of them to death, and make slaves of the rest, in order to civilize them and reduce them from their barbarous way of living. It is a very kingly, honourable, and frequent practice, when one prince desires the assistance of another to secure him against an invasion, that the assistant, when he hath driven out the invader, should seize on the dominions himself, and kill, imprison or banish the prince he came to relieve. Alliance by blood or marriage is a sufficient cause of war between princes, an the nearer the kindred is, the greater is their disposition to quarrel: poor nations are hungry and rich nations are proud, and pride and hunger will ever be at variance. For these reasons, the trade of a soldier is held the most honourable of all other: because a soldier is a Yahoo hired to kill in cold blood as many of his own species, who have never offended him, as possibly he can.
 

Taken From: Swift, J. 1967. Gulliver's Travels. Penguin Books: London.