In the short run, the problem is that it takes both (or more) sides to attain peace, and only one for war.

But peace is so much more conducive to prosperity that it is almost inevitable between democratic states. And democratic states tend to be strong, so once a country attains a democratic state, it tends to stay that way. So eventualy, most (not all, because democratic states do fall from time to time, so there will always be the occasional coup or demagogue that in one country or another) of the world will consist of democratic states where the politicians can't afford war.