Wars, the reasons why they happen and the effect they have are so intricate and complex that I seriously doubt that we'll ever see the whole picture. Everyone has their own personal agenda, and even the war for a "good" reason has a cause. One can for example say that WW2 happened because the German people were oppressed after the Allies had won WW1, and that WW2 was a reaction to this oppression. Hitler came to power because of how the "Allies" (Can't remember if the winning side of WW1 had a name, like in WW2, but bear with me here) treated the German people. But that's just one side of it.
Asking if war is good or not isn't the question that ought to be asked - we should ask whether it is necessary or not. War unites, and divides. Nothing will bring people closer than a common enemy, and nothing will bring them further apart than being enemies.
What we really need are for aliens to invade us, and give us a global sense of unity.
50% joke, 50% sad truth.