In my opinion, resolving conflict generally includes avoiding conflict. Political marriages have been around for well over 50-ish years and can be seen as peaceful resolution. Any sort of truce between two nations is conflict resolution. I would be willing to bet that as long as humanity has been warring amongst itself there have been people advocating peace and signing agreements.

None of these opinions are backed up by clever research projects, however. This is all from the top of my head.