I think this has been done before, but what would be the impact of Alexander Kerensky's republican Russia pulling out of WW1 after the February Revolution?
Interesting idea and might well have been better for Russia and possibly the world. Would have taken some time to organise but might well have seen a separate peace during say summer 1917. However I think the big problem would be how divided the country was. For the 1st part of the period up til Lenin's coup the main threat seemed to have come from discontent conservatives and militants. Most of them would have been enraged by this so I suspect a bitter civil war or at least some coup attempts. You might see several years of chaos anyway but might have still avoided the worst of Communist rule.
The CPs get a less gruesome version of Brest-Litovsk, the French and British press both lambaste the Russians as traitors, and Kerensky's Whites (presumably) prevent any Bolshevik revolution (especially if the Germans don't let Lenin through as OTL).
Possibly a lot less gruesome, for the Russians anyway. While L & H weren't the wisest I think they would have seen the advantage of a quick peace with relatively moderate terms. Also at that point, while Russia had lost Poland and some of the Baltics their army was still in reasonable state. As such if a peace could be agreed they might well get good terms. [Or if they tried then provided you avoid the army dissolving because the soldiers think peace is on the way those who stay will be motivated to fight harder if the Germans are insisting on draconian terms].
The western powers will be upset and kick up a stink but possibly not that much. No matter what happens in this conflict a friendly and reasonably strong Russia would be a good ally to have against a future German problem. Plus all that foreign investment cuts both ways. It was the loss of the Russian loans that turned Britain into a debtor nation as it loaned far more money to its allies, the bulk to Russia, than it borrowed itself. Similarly the French had a lot of investments from decades before. Cause too many problems and you give reason for the Russian government to be awkward or worse still collapse completely and all that investment is lost.
The CPs are then free to turn their full force on France and Britain. Does America still join the war? Who wins? And what will the postwar situation be like with no Red Menace?
America will still be formally at war before any peace is actually agreed in the east. How big an effect she will have depends on how long the conflict goes on and whether she will still drag her feet over actually fighting on the ground. It will be too late to avoid the French spring offensive and resultant strikes. What will happen with Britain's drive in Flanders knowing the Germans will soon have a lot more troops and how quickly Germany will seek to win in the west are the big issues. Will the Germans try something in the autumn of 1917 themselves, or possibly an even bigger thrust into Italy, although the logistics may not support that? Also depending on the details of the peace and circumstances inside Russia how much will Germany leave in the east. Especially if the government survives in decent order and Germany and Austria release their POWs they might have to consider a revitalised Russia re-entering the conflict.
Steve