Pancho Villa & Plausible Deniability
I think that Glass Onion is onto something; with the U.S. declaring war & launching a full-scale invasion of northern Mexico to stop Pancho Villa.
Say the 'government' of Mexico did become a Central Power ally to have a counter-balance against excessive American influence over their country. The Central Power could be an effective ally only if they won in Europe, which would necessitate that the U.S. didn't join the war on the Allied side.
Not wanting to commit national suicide, the Central Power-allied Mexican government couldn't directly launch attacks across the Rio Grande; but, what if Germany armed and I mean really armed Pancho Villa and gave him a mandate to cause as much chaos/distraction as possible along the border.
The Mexican government denies responsibility, citing that Villa is a foreign-armed free agent, but would welcome American assistance in putting down this 'lawless brigand.' If Villa causes major damage and a few atrocities against American civilians, the public outcry would compel Woodrow Wilson to deal with the Mexican threat first.
With no U.S. troops in Europe, Germany's 1918 Spring Offensive is moderately successful; the Allied forces along the Western Front are cut off from one another; the British Expeditionary Force isn't defeated, but is held off north of the Somme River. In OTL, the front came within tens of kilometers within Paris, close enough for 3 German Heavy Railway Guns to bombard Paris, creating some damage, but much more panic; in this ATL, the Germans push the French forces to within 7 to 15 kilometers of the city.
Even more Railway Guns are able to bombard Paris and cause real damage and the start of a mass flight of civilians from the city; thousands mistakenly flee towards the eastern roads out of Paris & hamper the efforts of the retreating French forces. Allied civilian & military morale plummets and the German morale increases immensely. Even more importantly, German industrial production, which had been falling, actually increases somewhat (in OTL it kept on dropping.)
North of the Somme, Douglas Haig is simply out of his element - he always was, but now it's evident after he tries to force a doomed to fail breakout plan upon the other leaders of the British Expeditionary Force. Canada's commander, Arthur Currie & Australia's John Monash outright refuse and in a rage, Haig relieves them of duty. However, when the news is relayed to London, British P.M. Lloyd George does the unthinkable, the relieves Haig of his command of the British Expeditionary Force and appoints Currie as the new head, with Monash as his Chief Of Staff. (In OTL, Lloyd George was contemplating this move if the war dragged into 1919 - the sudden near-collapse of the Western Front causes him to act earlier.)
Currie & Monash can't act immediately however, as their forces badly need logistical and manpower reinforcements and passive resistance from British subordinate officers smarting at being under the command of two 'colonials,' one of them, Monash of Slavic-Jewish ancestry.
As 1918 progresses, French morale plummets even further, with the ceaseless bombardment of Paris, grown even more intense as German intelligence realizes the toll it is taking on French morale. The French attempt to push the Germans back in their own counter-offense, but it fails. By September 1918, war materiel from the former Eastern Front territories ceded by Russia in the Treaty of Brest-Litovsk are finally starting to trickle into German industries; and even more importantly, a few more eastern territory occupation divisions are able to be move to the Western Front; a combination of many of the former Russian subjects feeling liberated by the Kaiser's forces and a steady flow of propaganda (mostly accurate), telling of the horrors of life under the Bolsheviks (plus a few massacres of anti-Kaiser activists and rebels) have all allowed a smooth occupation of the new eastern territories.
American troops start arriving in late October, but they are too little, too late. The British Expeditionary Force is still penned in north of the Somme, but is steadily being reinforced; the anti-war factions within British civilian and government sectors is getting louder and louder.
The Germans launch a Fall Offensive in late October 1918; Paris is entered and fierce street to street fighting erupts. The Germans are able to bring all of their artillery to bear on Paris and it is steadily pounded into ruin; several well-placed Railway Guns shells strike the Eiffel Tower badly damaging it; it completely collapses the next day and with it, the French will to fight; on November 11, 1918, The Kaiser's army takes Paris.
November 15, 1918: Currie and Monash launch the Somme Breakthrough. It begins with one of Currie's classic Creeping Barrage assaults, but at numerous locations along the Somme Front; German intelligence is unable to determine exactly where the breakout will occur and they know it WILL come - German officers who fought Currie at Vimy Ridge and at Passchendaele know the bombardment is merely the beginning of the storm. November 17 the British Expeditionary Force launches a combined tank, infantry and air assault on German defenses (OTL, John Monash was a proponent of the theory of what would become known as Blitzkrieg warfare; Arthur Currie perfected the tactic of the Creeping Barrage, and many tactics of modern warfare still used today.)
The Germans attempt to hold, but are badly mauled. By November 21, 1918 Currie & Monash are 30 kilometers north of Paris, when they are ordered from London to halt the offensive. Currie is furious, but obeys the order. Several factors have come into play: the anti-war movement in Britain has reached the point of huge public rallies and even strikes within crucial war materiel factories; intelligence has revealed that more German divisions have been diverted from the occupation of the eastern territories; news from the Admiralty has confirmed that shipping losses are becoming critical even with convoy protection and the new depth-charge weapon and that the French government has surrendered, accepting the Central Powers' terms.
The Armistice is signed on November 24, 1918; the Great War is over.