War is good because it's a way of balancing population. War often breaks out when there are too many young males and not enough females. This happened to be so during various wars and research will back this up (just don't ask me where you can find it since remembering sources is hard

). War will often take a lot of casualties and the numbers of young (aggressive) males will decline.
Also a decline of population will mean less mouths to feed and prevent starvation of overpopulation.
The black death was actually a good thing happening to Europe, and maybe you could argue the same with war. In medieval Europe large parts were overpopulated, people died from famine etc. Black death took the lifes of around 33% of European population, including a lot of scholars (most scholars were men of the church and those were the guys taking care of the sick). Black death rebalanced the population so there was no need to die from famine anymore and at the same time we see that many universities were founded to replace the scholars. Overal seen this black death had a positive effect on Europe.
A war might cause the same effect. Of course only if that specific area will not be drawn in a new conflict. Come to think of it since WW2 it has gone better for a lot of European countries. Western Europe has never been this prosperous, we have never seen a longer peaceful era in europe since the Roman Empire and education has never been this high.
Of course if you look on micro level war is a bad thing. Family members do die, soldiers do die, innocents do die, all true. But do you just look to the individual or to the common good? War can be something good. Ideologies 'we' support spread out and make this world a better place. The ideology of democracy and freedom is spread by the USA in both Afghanistan and Iraq now. Aren't those the ideologies the western world wants to be spread?
Lot of people will always say that war is a bad thing, but argueing the oppsoite might be interesting
Also do note that the above is not my personal opinion (although I do see the black death as something good for Europe, atleast if you look at the social-economic effects of it).