The decades after the first world war were a time of misery. The decades after the second world war were a time of peace and prosperity.
Between 1350 and 1950, there was at least one major military confrontation between European powers in every decade. What changed after 1950 is that we created international institutions.
There is a quip that nations either exchange goods or gunfire. Economic development and trade have been a boon to peace. The West also has institutions that link it together: NATO, the UN the IMF and the EU. These didn't really exist until after WWII and they do a great deal to explain why the time after the second world war was so starkly different from the time after first world war.
Just as the institutions of families, city councils, federal governments, corporations, churches and banks all deserve continual criticism and drive to improve them so that they adapt to changing realities and new possibilities, so do these post WWII institutions. They need improving. Without institutions we are like the other primates, though. We can't afford to neglect or discard them. And the West without these post-WWII institutions would be more like the world of Stalin, Franco, Mussolini, and Hitler than JFK and Reagan, Thatcher and Blair, Trudeau and Mulroney, de Gaulle and Chirac, and Kohl and Merkel.
The lesson of WWI is that without creating institutions that transcend nations we again slip into the the madness that goes by a variety of names (patriotism, nationalism, self-interest) and devolves into the worst kind of competition rather than raise us to the best kind of cooperation. It is a lesson too costly to learn twice.