This summer one hundred years ago, American troops played a notable part in WW1. For the first time, so as to protect the loans to the UK by the Democrat banking firm J P Morgan, the (Democrat) FedGov deployed military strength in a foreign war, intervening in a conflict which posed no threat to the homeland. That benighted policy has been repeated many times since, by Rs as well as Ds.
The "Central Powers" of Germany and Austria had achieved victory in the East; Russians had withdrawn and half a million men were moved West to help fight the French and British. Without the US intervention, I think they would have won there too; although both sides were near exhaustion, Ludendorff's "final push" brought his army as close to Paris as it had been in late 1914, and capture of the capital was imminent. That fine balance tipped in the Allies' favor because of American reinforcements. His army was pushed back.
The failure of that final push was, for Ludendorff, the end. He had nothing in reserve; supplies of materiel and food from Germany were drying up, as chaos and starvation hit the cities, aggravated by Bolshevik mobs. Meanwhile half a million doughboys a month were being shipped across the Atlantic, so there was no prospect of victory. He requested an armistice, for there was no alternative. The terms imposed, a year later, were savage; and led almost inevitably to Round Two in 1939.
Suppose that by 1918 America had been a country free of government. Then as a zero government society, there would have been no FedGov to deploy such force. The squabblers in Europe would have had to shift for themselves. I have no doubt that without hope of help from here, the Anglo-French side would have patched up a peace no later than 1916. As a result, there would have been none of the deep resentment that led to Round Two, and all those killed in the final two years of war would have survived it. If the Czar had joined the peace (and why not?) there would have been no Red Revolution. So the rest of the 20th Century would have seen no WW2, no Cold War, no Korean War, no Vietnam.
So would anarchism (even in just one country!) have brought rich rewards of peace and prosperity.
"But wait!" may cry the minimal-government cultists; "the same result would have followed had the FedGov merely pursued a non-interventionist policy!" And that's true, but how does one persuade a large power-hungry set of thieves and bullies to voluntarily restrict their reach? - with a piece of parchment? - Good luck with that. With an outcry of protest, then, a groundswell of opposition to the contemplated war? - Yes, that's more credible, but once a governing élite has decided to intervene, free speech is curtailed savagely, the First Amendment notwithstanding. LaFollete and Debs spoke scathingly of the supression of anti-war speech, and the latter was sentenced to ten years for his words. The Espionage and Sedition Acts of 1917 and -18 actually codified the prohibition of dissent and ordered the Post Office not to deliver mail known to advocate such views.
In the coming anarchist, zero government society everyone will be free to publicize any view they like about foreign wars, and of course volunteer to go off and join the fight if they so wish. I doubt many will. Perhaps those who have invested money in one side or the other - successors to J P Morgan - will want to don uniforms and place their bodies alongside their money. That would be a first. Much more likely, nobody will invest in the fortunes of any violent organization, foreign or domestic; for all will have been convinced that coercion is morally wrong; the ZGS will begin only when they have.
1918 also saw the FedGov corral all major commercial enterprise into its purposes, on the excuse that everything in America was to be directed to the war effort. The same was true of course in Germany, and t0 a slightly lesser extent in Britain. Competition was deemed wasteful (actually it's the only way that price-gouging monopolies can be prevented, and is integral to the free market) and there was no hurry at all, when that year was over, to resume normal market operations.
Ludendorff requested the Armistice, anticipating that the eventually resulting peace agreement would reflect Wilson's "14 points" which had some merit. Perhaps Wilson never did properly understand the ruthless, immoral nature of the political process; when the victors gathered in 1919 they were interested only in revenge, and Wilson retired from the process, his idealism in tatters. In the railway carriage at Compiègne, agents of the German government (Kaiser Wilhelm having abdicated) signed what was a surrender in all but name; the army retreated East of the Rhine and the Royal Navy continued to blockade supplies of food for Germans, so ensuring that the following July, the punitive terms of Versailles would be accepted for want of any alternative and so providing Germans with a powerful motive to correct the injustice.
So ended, on 11/11/1918, the conflict which began for no good reason - just the trivial one that Europe's governments had constructed some interlocking alliances that they felt obliged to honor. There was little actual hostility in 1914, and none at all among the various populations, who were busy trading with each other. It was promoted to them by their governments as "the war to end all wars" and that propaganda was as bogus as most State proclamations are, especially in war time. It resulted in the premature deaths of about 18 million people, of whom a third were not in uniform. This is what governments do.