War is the outcome of tension between two or more nations. In fact, wars do not happen instantaneously, but rather the outcome of long periods of tension and conflicts.
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.