three reasons for war (in my view)

1. social psycosis
2. need
3. ease

3. is the most common Germany WW1 and 2, Rome, Greece, Egypt, The Confederate Army ect These groups at the unset had better tech, training or strategy(leadership) and thus it is was very easy to conquer other nations/win battles.

In todays modern age you dont have the same circumstances there is no ease in dominating a nation (US Korea, Russians Afganistan). There is no need, lets say China decides to attack the United States, how does this help China, it doesnt the country would be worse off Econmicaly then if they had just went on with the status qou. Which leaves Social Psycosis I dont think this requires an example.

There would have to be a pardigm shift to change the world back to empire building through military might. A plague of epic proportions, a cosmic event, some sort of technichal glitch that destroyed machinary possiblities but unlikly. The super powers of the world are trending towards unfication by letting go of nationlism not bolstering it. Does this mean it doesnt exist, no but it is not the same as it once was.