WWI may not have ended all wars, but WWII may have, at least large scale wars between major countries. Some scholars are looking at war casualties since WWII and noticing the absence of large scale conflict. This period of time since 1945 to present is being called by some The Long Peace.

https://en.wikipedia.org/wiki/Long_Peace

http://www.correlatesofwar.org/

http://www.fallen.io/shadow-peace/1/