Off the top of my head, no perfidy, no use of prohibited weapons, particularly chemical ones, no targets which are not military targets.
Peace alone is not enough though, one major cause of war is *unjust* peaces.
Another major cause, though, is *incomplete* peaces, which is why Armistice Day led to a Second World War but Victory Day did not lead to a Third. (Which isn't to say that the Western Allies in 1918 should've overrun and occupied Germany -- that was not politically feasible -- but in a theoretical world of much greater resources the outcome would've been better.)
The Roman-Persian Wars lasted 681 years, and would've kept going if not for the Muslim conquest of Persia, which lasted 22 years. The difference is enlightening.
