The first phase, Operation Michael, was designed to drive the a wedge between the French and British armies, and force the British back towards the Channel Ports. Follow up operations (initially termed Operation Georg) would then turn north to deliver a decisive blow to the British in Flanders by capturing Hazebrouck railway junction. Meanwhile, smaller operations would distract and throw back the French, rendering them unable to aid the British and leaving their flank open for an eventual march on Paris.
However, lofty as these goals were, in reality they were strategic failures. While Operation Michael was a great success, driving the British back in a huge breakthrough, it lacked any strategic objective, and Ludendorff failed to set any major objective until it was too late. He reinforced success, rather than strategic goals, and only set Amiens as an objective after a week of fighting. After two weeks the attack fizzled out as the Germans outpaced their supplies, and failed to follow up their attacks. A miniature version of Georg (Georgette) was then launched to try and keep the British off balance, resulting in the Battle of the Lys, which also ended in failure. In the end this lengthened German lines by 80km, in a weak salient with no true strategic value.
They also lost over 350,000 irreplaceable troops. These soldiers were the best of the best, selected for their fitness and professionalism. By the end of April, these men were gone. The elite stormtroopers were depleted, along with the reserves that were supposed to keep the subsequent offensives on track.
However, the Germans came within a hairsbreadth of forcing their way through to the vital rail line at Amiens. If the Germans had captured Amiens during Operation Michael, they would have been able to severely damage the British ability to supply their armies in the field, and put a significant chink in the Entente's ability to coordinate. Follow up operations could then inflict local defeats on the Entente forces, and maybe, just maybe, drive them to the negotiating table before summer, and the arrival of the American juggernaut.
Let us assume that Ludendorff manages to rush everything in to strengthen his right flank and manages to press on to Amiens, strengthening his lines so we end up with a situation roughly like this:
Amiens is in German hands, and the right is slightly more stable, with a definite wedge driven between the Entente armies, with Haig and the BEF divided along the Somme, and the French First Army on their flank. The railroads are in German hands but damaged, and they have suffered heavier casualties and failed to take Arras, which is well entrenched.
The Entente now must marshal for a counter offensive, but the Germans cannot hope to continue their planned spring offensives. Assuming they attempt to, they instead opt to fall into the defensive by June, rather than continuing to attack into July of 1918. Ludendorff informs the Kaiser they must seek negotiations, but not an armistice, while the Entente is unbalanced, lest they find themselves with nothing.
Assuming for the moment the Entente accepts German negotiations, what might this potential peace look like?
We know from the German Septembreprogamme that they desired at the very least, annexation of territory in France, the vassalization of Belgium, the annexation of Luxembourg, and their existing gains in the East from Brest-Litovsk. In 1918, many Germans still thought some of this was desirable, keeping Belgium as a client state of the German Empire, and maintaining their gains in the East, and perhaps the return of some of their colonies.
Their success in the East may have them cocky, with Russia out of the war they could concentrate all their resources on a single front. However, as we know, that was not enough for a decisive blow and with the drip feed of American troops, the Entente lines will surely stabilize. The Entente will be throwing themselves at the Germans sooner rather than later, and even the Germans by now must know this.
Assuming this, we can say that Germany would attempt, at the very least to demand the return of their colonies, acceptance of the terms of the Brest-Litovsk treaty, vassalizaton of Belgium, and perhaps a territorial status-quo antebellum in the West.
We know the Entente will not accept Belgium being a satellite of Germany, nor will they accept a total territorial status-quo. France will at the very least be adamant about the return of Alsace-Lorraine, and Britain will not be moved on the subject of Belgium. They also desired to restrict German gains in the East, however, with the Soviets climbing to power, this may be more open to negotiation.
The issue of colonies will be a non-starter. The Kaiser may order the High Seas Fleet to sortie to provoke a battle sometime in June, and with the war still in flux this will probably not provoke a wholesale mutiny like it did in October while armistice negotiations were ongoing.Seeking to inflict a defeat on the British and get a better negotiating position (similar to the Dutch raid on Medway in 1667). Assuming that Hipper follows his plans to lure the Grand Fleet to battle at Terschelling as was planned for the October operation in 1918 then we can see this would most likely be a bloody climactic fight to the finish.
The British operators were well aware of those German plans though, and so we can surmise they would be well aware of the German plans here. If the High Seas Fleet sorties, the Grand Fleet will move to place themselves between the Germans and their line of retreat.
Now like a North Sea Operation Michael, this would be a last throw of the dice to try and force a decisive victory. Like Michael though, this will most likely end in a defeat for Germany, and the Germans further looking to come to terms. Germany will have lost her colonies.
Assuming then that this climactic Battle of Terschelling takes place in early July in response to a rejection of the initial German claims, the Germans now have to doggedly hold on on land. The front in July will look not too markedly different here, merely an absence of German advances, and the Germans preparing for what may be an inevitable Entente counter offensive.
With the Germans seeking negotiations, this will likely spell the dramatic collapse of their Central Powers partners.
Austria is beginning to fracture, and is losing ground to the Italians. The Ottomans and Bulgarians are only just hanging on. Emperor Karl may decide that his best opportunity to keep his throne is to throw in the towel completely, and if the Germans are negotiating, he will too. This would probably lead Enver Pasha and Tsar Ferdinand to conclude that they too should seek negotiations with the Entente. By August-September then, the Central Powers will all be looking for a way out, and the idea of peace will not be unreasonable.
The Germans then, will be hoping to attempt to negotiate by Wilson's Fourteen Points which have been laid out in January. They will naturally accede to the first four, but they and their Entente opponents will most likely begin to quibble over the remaining ten. The Austrians will begin to try and minimize the damage that will be caused in the empire, trying to prevent its break up. The Germans will use Belgium and Alsace-Lorraine as bargaining chips, while the Ottomans will most likely be forced to continue fighting in the deserts until they accept British demands.
One thing that will be immediately obvious is the Entente will demand, in exchange for an armistice, an evacuation of Belgium and France. This is the German negotiation power, so they will refuse, but if their allies accept, they may face the specter of continuing the issue alone. The Entente will still be preparing for an offensive, and if the Germans believe they can get more by evacuating, they will most likely agree to fall back to their 1914 borders.
Assuming then that in exchange for an armistice the Germans fall back to their 1914 frontier by September 1918, we would have peace negotiations opening up in October.
The Austrians will not have completely collapsed at this point, and the Germans will have smaller powers to use as debating chips to try and follow Wilson's remaining points.
What is undoubtable is that France will demand the return of Alsace Lorraine, Italy will demand Tyrol, and Wilson will demand an independent Poland. The British, who are already intervening against Bolshevism in the East, may be more amiable to Germany if she agrees to help against the Reds.
Serbia will be freed, and Bulgaria will be ordered to evacuate Romania with her Austro-German partners. Bosnia will fall under Serbian influence, and the Italians will declare a protectorate over Albania, but most likely not the Dalmatian coast as they desired.
In these peace negotiations Germany will be forced to make some territorial concessions. For one thing, the Entente will force the return of Alsace-Lorraine to France, and they will make a new Polish state in the East. However, I doubt the Kaiser would acquiesce to the surrender of all of east Prussia, and so some German territory will be ceded, giving Poland access to the Baltic, but much will remain in German hands. What is also notable is the Entente shall most likely not be able to force the return of Danish territory on Germany. Germany's biggest negotiating factor will be her ability to intervene against the Reds, something both Britain and the US both desire.
France will most likely be appeased with the return of her territory in Alsace Lorraine, and the enforcement of reparations on Germany and Austria. Romania will gain territory at the expense of Hungary, and Serbia will evolve into Yugoslavia, gobbling up Austrian territory on the Adriatic. Ukraine will be acknowledge by all parties, and Germany will most likely retain joint influence with Britain over the Baltics, perhaps amalgamated into one grand duchy for future carving up. Finland will already have achieved de-facto independence by this point.
Austria-Hungary will be the biggest loser, and Emperor Karl will most likely preside over the collapse of his empire. The Hungarians may hold with the crown briefly into 1919 to try and salvage some territory or legitimacy, but I personally predict the formal end of the empire by 1920.
The Ottomans too will be carved up according to the Sykes-Picot agreement, falling back on Anatolia, and most likely collapsing in the 1920s as well.
Bulgaria will lose her Macedonian territories and Agean coast to Greece.
All in all, Europe in 1919 will look very different to Europe in 1914. Germany bowed, but unbroken, Austria-Hungary divided up for spoils, the Ottomans in their twilight, and the Russian Empire fracturing into an increasingly brutal civil war.
The biggest issues in this post war world will be the dubious survival of the Austrian monarchy, and the containment of the USSR. The 1920s would be interesting, but we would most likely avoid the horrors of Nazism. The horrors of the Soviet Union though, could be on a greater scale.
All this of course, is not certain, and it is possible Germany fights to the bitter end in 1919. But as an entertaining "what if" this is hard to beat.