Create a scenario in which World War I is the last major war between nations for the next 100 years. No World War II, no Vietnam War, none of the defining nation-to-nation wars of the last century can take place. The only wars that are allowed are internal conflicts (civil wars, insurgencies, etc), but they cannot snowball into a war between two or more nations.
For those who thinks that makes no sense, despite the civil wars and global turmoil, the world we currently live in is regarded as the most peaceful time in world history, since no major nation-nation war has taken place over the last 50 or so years.