Dare I recycle a posting from a previous thread. My excuse is that it was not a believable answer to that problem but seems to fit here. OK, thank you for permission!
The Germans do not send the Zimmermann Telegram and divide their steel between tanks and U-boats, so the U-boat campaign starts slower. Thus the USA does not enter WW1 during the first half of 1917.
The German High Command decides that they should win if they knock Russia out of the war. In order to commit more troops against Russia, the decision is taken to give up territory in front of the Nivelle Offensive but not in front of the British because of the U-boat bases. Thus the Nivelle Offensive advances further and continues for much longer. Mutinies do not occur initially because advances are being made.
In the second half of 1917, German succeeds in taking the Ukraine quickly enough to obtain the harvest but the Russian revolutions occurs as OTL. The USA declares war anyway because of the submarine campaign. The Nivelle Offensive finally stops south west of Sedan leaving a German salient at Laon between the French and British armies. British offensives as OTL. Caporetto as OTL.
In early 1918, larger than OTL expeditions are sent to Archangelsk and Siberia because the French Army is trusted. These expeditions are still there many years later!
In March 1918, the Germans decide to attack the new French salient from north of Laon combined with a smaller attack from the Argonne. As an attack on the British front is expected, surprise is achieved. The appearance of German tanks is also a surprise. The French collapse catastrophically with some units refusing to go to the Front and those at the front surrendering. As the Russian revolution has occurred, the mutiny is more revolutionary than OTL in 1917. Verdun is belatedly abandoned after the Germans reach Bar-le-Duc and its defenders reach Toul without guns after suffering heavy losses from artillery. The Germans advance on Langres and again units at Troyes mutiny rather than be sent to stop the advance. Thus eventually most of the army to the east of Langres surrenders or retreats into Switzerland. A very strong British force is sent to help defend Paris and the Ypres offensive is resumed to take the pressure off the French. However, this weakens the British position and the Germans break through and take Amiens and reach the Somme estuary. Thus the British forces in the north become isolated and also lose Arras and Bethune. To the south, British and French commanders have lost confidence in each other and the Germans are now advancing rapidly. One general stated that the main problem slowing his advance was the need to guard the French prisoners. Thus the British evacuate their troops from France just as the first American divisions arrive.
In the second half of 1918, some British and American units reach Italy but the appearance of many, now very confident, German units causes an Italian collapse. The Germans manage to destroy most of the Italian armies in the north and take Rome and Naples as winter arrives. However, that is the high water mark. In 1919, a large Anglo-American army is deployed to hold the far south of Italy.
By 1920, the Anglo-American armies are capable of defeating the Central Powers if they could find a battlefield. However, the Channel Coast is well fortified and both Italy and Greece are mountainous. The German U-boat campaign had fallen from its successes of 1917 to a low level after convoys were introduced. However, at night the wolf pack tactic has now been discovered. The Germans are also trying to start a revolt in India.
As they fear a new encirclement, Germany refuses to withdraw from France or Italy unless the Anglo-Americans agree to demobilize their forces and the French and Italian forces are kept very small. The only thing driving the sides towards peace is that everyone fears the Bolsheviks. However, only the Anglo-Americans are fighting in Russia and cannot protect their communications enough to advance close to Moscow.
This might run to 1925 if we are lucky. Any further suggestions anyone?