i wrote a paper like this two semesters ago. my argument was that although war is a bad thing there have been leaps in technology during war time. during world war 1 radio technology greatly improved as the government took over the industry. i also argued that war is sometimes a necessary evil when you have no other choice but to fight. like when a bully tries to punk you, you have to confront the dude and if need be handle his ass. finally that war brings out the best and worst in people. You can look to hitler for the worst, and people like winston churhill and FDR for the best.
Here is why it's good.
-Economical benefits and companies like Blackwater get paid during wars. The country gets more jobs (building weapons, army)
US depression and World War
Also, ended slavery.
War is "bad": It is primitive, base, and below where humanity should be thinking at this point in time. To progress as a race we need to put war and permanently damaging conflict behind us. Needlessly, young men and now women, die. It makes rich people richer and poor people poorer. The later in more ways than one. By the by... I don't want my future technologies that are going to define how future generations live and humanity evolves to be forged during war time. Peace is the key to success. God. I really didn't mean for that to rhyme.
Good and bad things come from war. War in itself is not evil it is the intent behind the war the reasons you got into it in the first place. Its just like any other argument it happens because agreements can not be reached until one side is pushed to the point of asking for mercy.