First define how great nations win without having wars. Your thesis statement should be like this ...The supreme art of war is to subdue the enemy without waging war ..support your argument by giving real life example (the countries those that have adopted policy of peaceful coexistence )
coming to the body ..here first of all you will have to talk about ill effects of war and why these proved ineffective (as already mentioned in your outline )

like these ..
use of force causes nothing but devastes in both human loss and economic etc


now how nations win without fighting ?

by strengthening their economy (JAPAN ,CHINA etc)
by enhancing thier national morale
by adopting isolationtist policy UsA followed it and became a victorious nation

Now mention positive aspects of avoiding wars
Peace prevails
nations remain united as a whole
threat of non state actors is diminished


suggestions to make realize aggresive countries to follow the policy of peaceful coexistence

by CBMS
role of uno
role of civil societies and stake holders

conclusion:Let us wage a moral and political war against war so that we can put military spending and use that money for human needs

Wish you all the best .hope it may help you.