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.