While a German victory in World War Two has been discussed many many times in both academic circles and in alternate history fiction, a German victory in World War One have been relatively ignored. This is a shame as the early 20th century is a fascinating period of shifting ideologies.
The first part as with any alternate history is the how. There were two periods during the war where the Central Powers could have potentially forced the Entente to the peace table. The first of these was soon after the start of the war. The German army had remarkable success in its first push through Belgium into northern France. By September 1914 a month of the start of the war, the Germans reached within thirty miles of Paris. The advance stalled there as Germany attempted to branch out and surround Paris, but it is possible that a continued attack could have taken the French capital and knocked them out of the war.
However, the more interesting scenario is a German victory later in the war after the war had expanded in scope and the German goals in the war were fully established. This scenario surrounds the Spring Offensives of 1918, but likely requires some divergences prior to it. The main one would be the United States remaining neutral, which Woodrow Wilson pledged in his reelection campaign in 1916 but broke after the sinking of US merchant ships and the interception of the Zimmerman Telegram. The Russians were already knocked out pf the war, and the historical offensives came within 75 miles of Paris. If Germany had been able to take Paris and with no American forces on the ground, Britain and France likely would have accepted an end to the grueling conflict.
Now, if the Entente were defeated, what conditions would the Central Powers have imposed? Definite conditions would be upholding of the buffer States in Poland, the Baltic States, and Ukraine that were created from the Treaty of Brest-Litovsk with Russia, the return of parts of the Caucasus to the Ottoman Empire, and the cession of many African colonies including the Congo and Rhodesia to Germany. The Central Powers would also probably demand the division of Serbia between Austria-Hungary and Bulgaria and some reparations from France or the United Kingdom. The Ottomans would also likely get Libya back from Italy since it is only six years since they lost it, while Egypt would become independent from British rule with German advisors. In Asia, Germany might receive French Indochina if France was soundly defeated. There were also aspirations for Afghanistan to join the Central Powers and for an independent India. However, the likelihood of these happening is suspect, at least in the short term. An expansionist Afghanistan and Indian rebellion could happen further down the road, but how and when in uncertain.
With the concessions set, we can now look at the immediate effects. The primary concern for Germany would be the Russian Civil War and keeping their east European buffer States in line. Although Germany did help the Bolsheviks by helping Lenin return from exile in 1917, they would not ultimately want a Communist state right next door. There would be at least some German and Austrian intervention in the civil war on the side of the Whites along with an American contingent in Siberia. However, just as is our history, it would not be very successful. Germany’s economy would be booming with no reparations and deindustrialization, but they would be preoccupied with their buffer states to send a significant force. So Poland, Ukraine, etc. will probably be economic puppets of Germany, but the Reds will likely still come to power in Russia.
What happens to Austria and the Ottomans are more tricky. Both are on the victorious side, but now have to contend with ethnic instability and nationalist movements that had been present since the mid-19th century. There is certainly going to be unrest in newly annexed Serbia as well as rebellions in Bosnia and possibly Hungary. One path that could result from this is the creation of a third kingdom within Austria-Hungary for the Slavs. This was considered before the war but the assassination of Franz Ferdinand ended any consideration of the proposal. It could placate the Serbs and Croats, but it could also speed up Slavic pan-nationalism and lead to the collapse of the empire.
With the Ottomans, they will face many uprisings including the Arabs which had been fomented by the British during the war, the Kurds, the Armenians, and possibly Egypt if they get expansionist or if Egypt is ceded to the Ottomans in the peace negotiations. For the Sick Man of Europe, the victory in the war would have been a boost, but would only be postponing the empire’s downfall.
It is difficult to definitively speculate on how the world would develop beyond these short few years. There are just too many possible outcomes for each country with the rise of so many different ideologies. France would continue a succession of weak governments that plagued the third republic and could fall to an extreme ideology, but if so it could be any of Communism, some right wing fascist analogue, or even a revival of the monarchy is not out of the realm of plausibility.