The negative effects of war are, of course, death and destruction. Millions can die in big wars like WWII. Cities can be devastated, hurting a country’s economy. These things cause terrible human misery.
There are, however, also possible positive effects of war. War can correct terrible injustices. For example, the US Civil War caused an end to slavery in that country. World War II ended the Holocaust. Wars can also lead to technological advances that help society. One instance of this is the development of air travel after WWII, which was strongly influenced by the wartime development of large, long-range bombers.
Contrary to the common belief that war has only negative effects, it also has some positive impacts. However, whereas the negative effects of war are the same regardless of the place of war, the positive effects may vary depending on where the war occurred. The most common negative impacts of war include loss of human lives, economic losses due to destruction of capital as well as disruption of trade, human suffering, the spread of diseases, displacement of people and destruction of the environment, among others.
On the other hand, war can also impact the economy positively in the short term as during the Great Depression during the 1930s in the United States. The war mobilization efforts and military spending stirred capacity utilization and reduced unemployment through increased army conscription. Technological advancements are also a positive impact of war. The railroad networks of the Europeans for instance, that had a significant military strategic input in terms of countering enemy attacks, also helped the civilians. The GPS system which was specifically developed to enhance navigation by U.S military during the 1990’s has found great use among the civilians to date. The First World War helped establish machinery that would help maintain world peace. The League of Nations was founded after WWI as a platform to settle disputes peacefully and even though it has been criticized for failing to avoid World War II, it set precedence for the formation of the United Nations. Some wars have also led to the end of despotic leadership in various countries across the world such as the Libyan civil war that led to the ousting of Muammar Gaddafi in 2011.
In conclusion, even though war has negative effects, it also has some notable positive impacts.
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.