Take Europe for instance. For thousands of years war has broken out between the nations it's even triggered wars that escalated across the world. One hundred years ago, peace across Europe would have seemed an impossible dream.
However, now it's completely the opposite. War now between the nations of Europe seems ludicrous, I doubt anyone could imagine England invading France for instance, the region has never known such a stable time before.
There's many reasons why this is, such as increased communication, open borders, exports and imports, a single currency, immigration, emigration. People now visit other nations, know and communicate with people throughout the world and relocate to states their forefathers would have known as enemies.
War between nations will end because the concept of a "nation" will not survive. Eventually we will all be one people.