I've been re-reading Thomas Fleming's outstanding book on WW1, The Illusion of Victory. It's a large work, compiled by a meticulous historian who combines mastery of the wealth of facts with a rare narrative ability that makes it hard to put down.
There are a few minor flaws (twice, Fleming tells us that the Constitution can be amended by a two thirds majority of the States, instead of the correct three fourths) but in the main it's magnificent. I was particularly interested in whether the American Expeditionary Force (AEF) really tipped the balance in the Allies' favor, as is often said; for by the Fall of 1918 the AEF strength was but a tiny fraction of the combined British and French armies on the Western front. Seems to me the key was that from mid-1918 onwards, more than 250,000 men a month were being shipped across the Atlantic to augment the AEF. Most of them saw no action, because Ludendorff recognized that these newcomers would inevitably break the stalemate. After a final and almost successful "push" for Paris came to nothing and while back home in Germany starvation was raging and Bolshevists rioting, he declared the German situation "hopeless" and an armistice was requested.
That "war to end all wars" (ha! wars will end only when governments do) began by accident. The even worse one 25 years later also involved a lot of blundering, but Hitler did intend to wage it to his East and FDR certainly intended to drag America into the fray; but in 1914 the "European powers" built a web of alliances designed to prevent war, and were horrified when it failed. Even when Wilson led the US to join it, he was reluctant.
Blame can be spread around liberally, but to my mind the Russian Tsar, Nicholas II, bears most of it. Nobody else was poised to invade his country, yet he declared war on Austria. His reason was that Austria had invaded his friends in Serbia, which was not a party to the web of alliances. That Austrian action was highly dangerous, but it broke no such agreement. The Tsar did, and later paid dearly.
So it was a Russian who began it, and ironically it was Russians who ended it; for by 1918 there was a new government there and it deliberately encouraged Bolshevik revolutions elsewhere, notably in Germany. Those uprisings, combined with acute shortages of food resulting from a British Navy blockade, created chaos back home at the very time when the Army on the front was doing at least as well as ever. Ludendorff abandoned hope not because of a military setback - of those there had been plenty, since 1914 - but because his entire support structure in Germany was collapsing.
All that is rich in terrible irony. The Germans had helped Lenin and his friends reach Petersburg to foment revolution in 1917, and once it succeeded Lenin pulled Russia out of the war, so freeing up German resources to fight in the West. But then the revolutionary fervor spread back to Germany and so prevented that fight succeeding. The Communist viper turned on its benefactor. Hitler did not forget.
The 4-year slaughter was accidental in another sense: had the British government not joined it in 1914, it would have been over far sooner - probably by the end of that year; and Britain was not obliged by treaty to join. She had an informal understanding with both France and Russia to come to their aid if attacked, but the decision to intervene was optional; and it was almost a toss-up. Had not Winston Churchill, then a junior Minister, passionately argued in favor of intervention Britain would have stood aside, the Germans would have reached Paris before Christmas, defeated the French exactly as in 1871, and then gone home.
Facing the whole weight of Austro-German military might, the Russians would have found a way to back down, the appalling conditions of 1917 there would not have developed, so the Bolsheviks would never have won a revolution, and the 20th Century would have had a radically different history. But Churchill prevailed, and the fateful chance was taken, to disable the German continental challenge to British worldwide power. It paid off, but the cost was a near-crippling of her Empire. Wilson was by then too sick to grasp the significance of that, but it was not lost on the young Franklin D Roosevelt, who engineered a replay in 1941 and displaced that Empire once and for all.
The chapter of accidents was completed by the US entry into WW1. There was, obviously, no defensive need for it; there was no popular will for it, and with a large minority of the US population being of German descent none was likely to arise. Further, Woodrow Wilson was an academic and idealist, who had no political ambitions to be a "War President." His 1916 re-election had succeeded largely because he had "kept America out of the war", so what changed?
J P Morgan & Co (JPM Sr had died in 1913, so Jr was in charge) had led a major investment in arms for the UK. If the Anglo-French side should lose, that massive funding would be in peril; so Morgan, a generous donor to the Democratic Party, lobbied for US military intervention to secure his money.
Further, that money never actually left the US; it was spent on manufacturing armaments here at home, and the hardware was what got shipped to Britain. So by 1916, not only was Morgan's money at risk, but so too were millions of good jobs in cities across the country. Democrats would not survive the 1918 elections if those were lost. Those accidents of domestic politics drove Wilson to intervene, at a cost of 120,000 American lives.
Did the intervention help end the war? - yes, as above; the huge flow of US soldiers was one important factor that caused Ludendorff to give up. But in another sense, no; since the very day the war began, so Fleming explains vividly, the UK Government had been actively campaigning for US help and after the Lusitania was sunk in 1915, hope of success was very lively. Had it withered in the face of resolute US neutrality, there can be little doubt that an end to hostilities would have been negotiated very much earlier; there were repeated German approaches, all of which were rebuffed, and the terms of such an agreement could by no means have been as vicious as those actually imposed in 1919.
So a firm US neutrality would actually have ended the slaughter much earlier, saving perhaps eight million lives, and would have provided no casus belli for the much larger slaughter of WW2. Such is the cost of "accidents of domestic politics." In the coming zero government society, none will occur - because politics will be in history's ashcan.