A lot has changed since then. Civilised nations have understood that militaristic conquest is useless
People in 1914, and then in 1918 also wrote many fancy and beautiful things about how war is a thing of the past. The Americans, under Woodrow Wilson, got so idealistic - they even insisted on the creation of a whole new global organization called the League of Nations - where all civilized people would meet to discuss their problems in peaceful fashion.
What a fantastic failure then, when this "War to End All Wars", ended up leading to an even more disastrous World War. After that war, people also swore off conquest, and created yet another new organization.
This organization similarly failed miserably at its mission of keeping the world conflict free.
The only thing that is keeping the "civilized world", as you say, from eating each other like in the past, is the very, very uncivilized nuclear weapons that have entered the scene.
That's what truly made conquest between large "civilized" nations "useless". The American nuclear umbrella, allowed Adenauer and De Gaulle to get over their mutual paranoia, and found the project which is now the EU. Otherwise you would still have France occupying large parts of the Rhineland, and Germany looking for ways to get friendly with Russia/China (more than now).
So what makes your world civilized is not that human beings have suddenly changed, but that there are now some very uncivilized weapons in existence which have changed the risk calculus.