First, cease and desist any further harm to anyone else! No more fatalities, casualties, or tragedies!
Then, the two leaders sit down like the adults and role models they’re supposed to act like and talk it out. Come to a mutual agreement and understanding, vowing to assist each other in some form or fashion.
The remaining world leaders should get involved under one condition:
They all support both countries by allowing them to shake hands and settle matters solely between one another while commending them and offering equal support for them as well. No slick or underhanded moves or attempts, otherwise, justified harsh consequential penalties would follow!