As long as possible with it still occurring, or as long as possible, into perpetuity if possible?
I would say break down the alliance system, have no countries be able to rely on one another, so that the leaders think twice about igniting a regional or even global conflict out of fear the rest will descend upon them to feast upon their corpses.
That, and increased globalism and political pluralism over time.
Then keep moderates in power (or at least non-warmongers) and you have a situation where the benefits of a conflict no longer outweigh its costs, and the leaders of the Great Powers of the day will know and accept this.