What made WW1 a very schizophrenic war is the substantial number of soldiers who hoped that the enemy would win and / or that their empire (and all empires) would loose:
Irish fighting for the UK, Alsatian-Lorrainers and South Jutlanders fighting for Germany, Czech nationalists and Italians fighting for Austria-Hungary, Poles fighting for Russia, Communists in all countries etc.
Good point. It's funny to consider nationalism as being one of the major factors for the start of the war. But I suppose we should separate the terms "nationalistic/patriotic" and "loyalty" just as we separate the views and actions of one's government from its citizens/subjects. I guess the divergence is best exhibited by how the governments of Europe simply were not keeping up with the rapidly changing times; culturally, political, technologically. The people were forced to rally around each other rather than their governments and were naturally suspicious of power and privilege during times of military and economic strife.
We see the United States was also in the midst of a progressive era at this time. One Democratic President served for eight years between 1913-21 and even his two predecessors who combined to occupy the White House for the first thirteen years of the century (Taft, Roosevelt) were decidedly left-leaning and progressive by modern Republican standards. The government of the United States proved far more flexible - both in the sense of its political apparatus, and in terms of the individuals of the era who led it - than the conservative monarchies and post-monarchies of Europe.
Of course the government of the United States wasn't tested (not at least since the Civil War) like that of England, France, Germany, Russia, Austria-Hungary, Ottoman's, etc. Nor was the US ever in any serious jeopardy of winding up on the losing side of the war. Public perception is influenced heavily, not only by the specific conditions unique to individuals themselves, but by broader perspective. Citizens and soldiers alike need hope and to be reassured by the likelihood of ultimate victory. When hope of this is lost, as it was in Russia by the time of the Tsar's abdication and in Germany not much later, people will often choose to pick a fight they can win. Can't beat the Germans? Take down the government. Can't be the allies? Depose the Kaiser.
The oft used phrase in sports is that "winning cures/takes care of everything". Does this not work for societies and governments also? Does anyone think that Nicholas would have been forced to abdicate had that Russian steamroller flattened Germany within a year or two as initially suggested? Maybe Nicholas's downfall, like that of a number of politicians, was not being able to remain popular and a source of national unity while tempering the lofty expectations put forth by his people...Of course you generally win elections and/or stay in power by shouting "Yes we can", not "No we shouldn't".