"The war that will end all wars" is a fairly well-known phrase referring to World War I, especially for the tragic irony that can be read there in hindsight.
But is it really a phrase uttered at the beginning of the conflict? By whom? Was it a propaganda phrase to convince people to fight, or did it contain sarcasm even then?
The originator of the idea was H.G. Wells, in his monograph "The War that Will End War" 1914 (text):
It aims straight at disarmament. It aims at a settlement that shall stop this sort of thing for ever. Every soldier who fights against Germany now is a crusader against war. This, the greatest of all wars, is not just another war—it is the last war! England, France, Italy, Belgium, Spain, and all the little countries of Europe, are heartily sick of war; the Tsar has expressed a passionate hatred of war; the most of Asia is unwarlike; the United States has no illusions about war. And never was war begun so joyously, and never was war begun with so grim a resolution. In England, France, Belgium, Russia, there is no thought of glory. We know we face unprecedented slaughter and agonies; we know that for neither side will there be easy triumphs or prancing victories. Already, in that warring sea of men, there is famine as well as hideous butchery, and soon there must come disease. Can it be otherwise? We face, perhaps, the most awful winter that mankind has ever faced. But we English and our allies, who did not seek 15this catastrophe, face it with anger and determination rather than despair. Through this war we have to march, through pain, through agonies of the spirit worse than pain, through seas of blood and filth. We English have not had things kept from us. We know what war is; we have no delusions. We have read books that tell us of the stench of battlefields, and the nature of wounds, books that Germany suppressed and hid from her people. And we face these horrors to make an end of them. There shall be no more Kaisers, there shall be no more Krupps, we are resolved. That foolery shall end!
That was in 1914. British prime minister David Lloyd George was not impressed, supposedly quipping in private that “This war, like the next war, is a war to end war.” Others echoed this sentiment, but by 1914 there was little choice but to fight, and to pour every ounce of material and every breathing soul into the mess that had been created.
There is an interesting article by Nordlund on HG Wells and his thoughts in 1914 in Modern Intellectual History 15.3 , November 2018 , p747 - 771, in which it is argued that HG Wells was being much less prophetic and much more pessimistic in his words than most now realize.