The war makers had let the genie of nationalism out of the bottle and the peacemakers could not put it back.

From The Long Shadow: The Legacies of the Great War in the Twentieth Century by David Reynolds.  Page 15.

The declarations of independence in 1918, validated at the Paris peace conference, made the so-called national principle the prime test of state legitimacy, rather than dynastic inheritance or imperial rule. Here indeed was a “seismic shift” in European history. Yet the principle of nationalism was an artificial construct, almost an anthropomorphic fantasy. Consider some of its cognate terms—national consciousness, national will, self-determination—in each case the nation is treated as analogous to an individual human being. But this postulates a unity and coherence that does not exist in any state, certainly not the new “national” states born in 1918. In any case, by the 1900s the person, the self, was understood in much more complex ways through modern psychology. Viewed in darker light, the national “self” seemed like a bundle of unconscious herdlike instincts that needed to be controlled through international institutions. Whether nationalism was a blessing or a curse lay at the heart of debates about peace and security in the 1920s and 1930s. Although nationalist frenzy was more consequence than cause of the Great War, the war makers had let the genie out of the bottle and the peacemakers could not put it back.