Hmmmn.
Given how everyone apparently wants peace anyways, why don't we trick the world into it?
We cut a deal with each of the elemental nations (without them knowing about the rest) about creating a fake 'Hidden Heaven'. A massive Big Lie, which will let them force a complete ceasefire for at least much of a generation.
Hidden Heaven would be a group that has, supposedly, been living in the clouds using seals for untold generations. After 1,000 years they have returned their gaze upon the elemental nations, and have found them wanting.
Then we have every Kage pretend that Hidden Heaven has decimated their ninja somehow and so everyone has sworn fealty to them now.
Of course, at first, every kage believes themselves to be the sole ally/creator of this false Hidden Heaven. Then once everything is actually implemented, it becomes clear to them that they've all allied, and so have been conned into an alliance between all the elemental nations, creating peace.