Also America could turn Communist or have some kind of revolution break out
Hmm, that has potential.
OK so the US elects someone stupid enough to get involved in WW1 earlier, but the Germans (and allies, maybe give then Italy, Sweden and the Netherlands) also do better. France and the UK hold on, with both sides taking heavy casualties while the Russians are pulled back and do scorched Earth. The European theatre is stalemated after the Germans dig in, with occasional mass wave attacks and lots of dead. The US mobilises slowly, with significant resistance to the whole "Go Die in Europe for the Greater Good of your Masters' Investments" idea, and conscription. Potential for a spot of civil war.
The Germans ferment revolutions in Russia and Finland (a la the Grafton Affair but with less booze). The Russians deal with this in the time honoured manner of mass executions, burning of villages and blaming the Jews. Lots of pogroms and the rise of a "windy mixture of nationalism, monarchism and, above all, anti-Semitism," (as Professor Muller will no doubt describe it).
By late 1915 the US troops start arriving in France. They're poorly equipped (war profiteering), badly trained and fed into the meat-grinder piecemeal. Casualties are horrendous and some US units break. This reinforces the UK/French opinion of US troops...
The Dardanelles happens and goes just as badly as historically.
Then in 1916 Ireland erupts. Let's say Redmond was murdered in '14 by a Unionist and far fewer of the volunteers ended in France. So the IVF is several times it's historical size (give it 50%, so ~100,000). The British decide to introduce conscription and also disarm and suppress the IVF. So Easter doesn't see s "nine days insurrection" but a major revolution with many times more revolutionaries and an emphasis on guerilla warfare. They're equipped by Germany with lots of captured Russian rifles, machine guns and even some light artillery. British naval bases are isolated and neutralised, with the assistance of German naval support (some AMCs mainly). There's also a US volunteer force, not large but a few hundred strong.
Britain attempts to bring in more troops, made difficult by Germans subs operating around the coast (and basing at some smaller ports). Troops from Ulster are deployed and the rebellion gets nasty with all the expected atrocities and reprisals. This doesn't help[ the UK's food situation either.
Mid '16 sees the UK (and France) getting desperate. The lack of reinforcements (due to commitments in Ireland amongst other problems) hampers the resistance to a German attack, which is assisted by the German use of a mix of Blue Cross and Green Cross gases to which Allied respirators provide no protection. After a couple of months in the line a US division breaks after >50% casualties and retreats, the British start shooting deserters and a mutiny breaks out. The Germans are smart enough to handle this sensibly and bypass the US forces.
The French troops see this and morale suffers badly. There's talk of a negotiated peace and mutiny.
Meanwhile in the US it's a mid-term election year with all the fun that entails. Helped by active attempts to stop anti-war/socialist candidates with the police breaking up meetings and engaging in the usual measures to prevent the wrong people winning. However there's a continual flow of casualties home. The news of the mutiny in France hits just before the election and polarises opinion. There's pretty open warfare in the streets, especially in the industrial cities and the socialists are organising self-defense militias. Irish-Americans join them and the socialists win big in the elections. Only to have several reps and senators arrested during a major crackdown on "sedition". The socialists (who aren't all socialists but let's just call them that) resist and the warfare escalates. Troops are deployed and while some fire on the crowds others don't. A civil war, in all but name, begins.
After significant German advances, and horrible Allied casualties, the lines break in the Western Front. A local cease-fire is arranged (the Germans are badly mauled and exhausted). Both the UK and France are facing mutinies far worse than Étaples.
When Wilhelm demands further attacks the German troops are also reluctant; after some 'incidents' get out of hand German troops also start talking mutiny.
In Russia the repression isn't working, the cities especially are a powder keg and Germany starts lobbing matches; arms and agents ferment revolution. When the revolution does break out more units remain loyal to the Tsar and a vicious civil war breaks out.
In 1917 all combatants are exhausted. The UK has lost Ireland (and gained a lot of Unionists via ethnic cleansing) and is facing a revolution of it's own. France is falling apart. Russia is ablaze. The USA is coming apart.
Just in time for the Okie flue to spread....
For added fun assume someone figured the potential of anthrax and decided to use it.