Lets say the United States attacks Canada. Would it be right for Canada to let this happen? By defending itself, a war would erupt between both countries, but Canada would be achieving an act of self defense. Examples like these have been usually the main cause of wars, and only until recently have the subjects and reasons begun to change. Yes, it is unethical to attack an other country for territory and power, but that is not considered war. In the example, once "Canada" reacts, then war would be officially declared. This scenario would be ethical because Canada would be defending itself by using war as its excuse. One of the only ways for humans to force their opinions on other groups is to use war. Using actions like peace talk and political disputes cannot force other countries to submit to any rule. Deals are usually cheated, and because war has few rules, it is the only way for groups to force an action on others. After a country loses a war, it is almost impossible for them to deny any influence from the winner. This is why war is right because it is usually an expression of defense and human argument and decision. It helps develop society and it makes the final decision on what changes should be made in the world. War is for the greater good, because when changes in society are necessary, sometimes humans need to fight for what is right.
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Just ask the winners. If we look at war in history, the ones who wrote about it were the winners. Nobody really cares about the losers side, because of the idea that if they were right, they would have won. Hypothetically, if we had lost the Revolutionary War, it would be George Washington not Benedict Arnold who's name would be synonymous with being a traitor. After all, had we lost, he would probably he hung for treason. Had Hitler won WWII, instead of being looked at as a horrible man, he would likely been heralded as one of Germany's greatest leaders.
The question of positive and negative effects of war is primarily dependent upon the nature of a particular war, but one can argue with certainty that all wars have disastrous effects on both the combatants and civilians, and a few wars have some positive outcomes. The issue might be better phrased as, "Is a particular war justified by its potential benefits?"
In World War II, for example, the world was facing an ideology in Europe--National Socialism ("Nazis")--that destroyed and would go on to destroy, if successful, an uncountable number of people based on racial, religious, and medical characteristics. The approximately 8 million Jews who were killed by the Nazis would have been only a minor number had the Nazi regime succeeded in its goal of controlling western and eastern Europe. To stop this movement, those who opposed the Nazi ideology went to war, but in the process of defeating the Nazis (and the Japanese), a staggering number of civilians were killed, wounded and displaced. In addition, the number of killed and wounded combatants, on all sides, is staggering. Although one can argue this war had to be fought, the human cost is actually unknowable.
All wars have benefits, the principal two being technological advances and advances in medical science. War accelerates both technological and medical advances because all sides are attempting to invent better ways of killing and better methods of preserving the lives of combatants, and some of these advances benefit the civilian populations, assuming they survive the war.
It is reasonable to conclude that all wars are bad, but, in rare cases, war is forced upon the participants by the actions of one or all parties. It is also reasonable to believe that the negative effects of war, in all cases, far exceed the potential benefits. The fact is, there are no good wars. There are only wars that are less bad than others. If you interviewed a thousand combatants in various wars, you would find it difficult to find any who would want to go through combat a second time.