According to the Rise and Fall of the Great Powers, Foch's September 1918 counteroffensive involved "102 French, 60 British Empire, 42 (double-sized) American, and 12 Belgian divisions."
So if there's half a million Americans (instead of 3.8 historically mobilized - note, mobilized, not necessarily "in time to see action") over there, how much difference does that make?
We have had several thread that are related. It looks a lot like the "USA does not enter war" but without the financial down sides for the Entente. We avoid ammo shortages and financial crisis of April 1917. We will have a severe effect on morale eventually and greatly strained diplomatic relationships, but after all, the UK did not really want to attack in France until 1916, so the USA is just doing the same type of time scale. So lets assume the French/UK can handle morale until through 1918.
1) Sea War. Much the same but a wee bit better than OTL for Germany. USA still sends enough ships to make BB on BB battle suicide for Germany. USA still seized Dutch Merchants. Rationing laws still in effect. Still build a lot of ships. We still see the North Seas/Scotland mine field. I see a decline in the number of USA escort ships, so the losses are higher than OTL. But I don't see it having a huge impact on the war, just less food and consumer items in the UK. I could be missing a detail here, but generally, I have existing escorts turning the tide in the U-boat war.
2) 1917 Land: USA did not have a lot of forces outside of our symbolic 40,000 man 1st Inf. Without the draft, it will be 14,000 man division in France in 1917. War unfolds much like OTL. Now we have to think about how the USA fighting half heartedly impacts the Russians. If you are doing an ATL, you have to make a call about do the Russian make an early peace? Yes/No. Same for its 1917 attacks. Yes/No. This could be dramatic impact on history, and if you choose Yes, the ATL will rapidly diverge from OTL. Since the USA did not start increasing the size of its army until after July 1917 OTL, I would go with no on both. But I can see the argument that the Russia makes peace early. You need a Russian specialist on this one who knows how they were making decisions. It will be when the draft bill fails (July 1917?) that you have to do a morale check for the Russians.
3) So we get to early 1918. For my TL, i generally assume force changes of less than a corp had a small impact and I think this is a good rule of thumb. In most cases where the commanders have say 10K more or less men in their multi-million man armies, they make the same calls. Land forces for both sides are within one corp of OTL. The U-boats might have gotten a few more supply ships but the USA is using less ammo training. Call it a wash (plus or minus 2%).
So we start OTL Spring offensive. I have not gone over this in great detail like I did with Verdun and before, but I have look at the higher level. When the attack first starts, the USA did not have a huge number of men in the line. You can see butterflies one way or the other, but it likely stalls much like OTL. The problem comes when you get to the hundred day counter attack. Generally speaking, the number of American troops in theater is about the same as the number attacking in the hundred day attacks. The USA will have 10% to 25% of OTL levels. Better troops. Lavishly supplied by OTL standards. So the allied commanders have to make a decision. They will want to counter attack but they will have to use all of their reserves. Again Yes/NO. I go with no.
4 Yes). The attack starts out with OTL strength. It will have like OTL success. The problem is they will run out of reserves well before the 100 days of OTL and the Entente will not really go to ZERO divisions in reserve. So we end up with two powers too weak to attack in late 1918. Both will try to bring up more men in 1919. A negotiate peace makes sense like it did each of the preceding winters. We then have a series of ATL questions that have a big impact on what comes out. I go with the most likely result being in order peace terms, Germany/France both effectively collapse, German makes unfavorable peace, and German gets favorable peace.
- Was A-H collapse due more to food or setbacks in France by Germany?
- The UK is getting low on manpower. Each time it does more conscription it has problems. Ireland, Canada, etc. The UK can get more men from within its empire, but at a price. Does it pay the price? For example, the Boers want concessions on Indians workers in the mines. Various princely states want things.
- If so, how fast how many?
- Will France even be willing to attack in 1919?
- Will the UK pull troops out of secondary fronts?
-etc.
4 NO) I favor this option as most likely.We then have have to repeat the analysis in the above section. The changes are IMO that A-H is a lot more likely to stay in the war and it will be much, much harder for Entente to launch a grand offensive in 1919 after a better part of 9 months. Also, Germany still has 'ample' reserves. I see a negotiate peace as much more likely since in some ways it will resemble the phony war of WW2. Very tough on internal politics.
Now I have left out the secondary fronts. I do see the Ottomans as benefiting a good bit. IMO, in 1918 as USA troops are not arriving we will see the UK go on the defensive in Iraq and Palestine. In both locations, it is easy for the UK to fall back towards newly prepared defensive positions and free up divisions. If the UK destroys the RR and road improvements as it retreats, it will be hard for the Ottomans to attack.