Starvation in Germany. Ration allowance for German civilians had fallen below 1000 cal/per person per day, and the ration allowance was not being achieved in any of the large cities, because the distribution network had totally fallen apart, and rural areas weren't sending stuff to cities (to avoid starvation themselves). Malnutrition and diseases from malnutrition had become endemic. Numerous diseases were rife. It has been estimated that around 1 million German died of starvation or diseases resulting from malnutrition over the course of the war. This was on an accelerating path; insignificant numbers in 1914, a couple of hundred in 1915, a few thousand in 1916, tens of thousands in 1917, hundreds of thousands in 1918. Fighting on into 1919 means, conservatively, 3 million dead civilians in that year, possibly up to 7 million dead civilians. That's from food shortages alone.
Revolt. The civilian population were pissed off with the war. Socialists were actively fighting with the authorities to bring an end to the war. Outside of the Conservative Establishment, the workforce and the people wanted an end.
Mutiny. By the end of 1916, the Bavarians and Saxons in the front lines were already pointing out to the British Army where the Prussians were, and telling the British Tommies where and when Prussians would be launching trench raids so that the Tommies could kill the Prussians and maybe they could all go home. This was getting worse, and by the Black Day of 1918, this had reached serious proportions. Forget the "mutinies" of the French Army, the German Army post August 1918 was essentially broken.
Defeat. The manpower had shifted irrevocably towards the Entente. France had been bled as dry as Germany, but Britain still had a way to go, and American troops were arriving in ever-increasing numbers. The Russian gains had been used in March 1918. They're now dead. The British and French Empires were still in the hunt. The Ottoman and Austro-Hungarian empires are down and out, so the Entente will be able to relocate the forces used against them (Palestine, Salonika, Mesopotamia, and so on). That's another 2 million veteran troops available for the Western Front, when the German Army was done to well below 1 million.
Add in to this the technological advances (tanks, combined arms, etc), and the German shortage of materiel, and there's only one way that continuing the war in 1919 will end, and that's sausages in Berlin by June at the latest.
Economic collapse. Britain and France were struggling to meet the cost of the war. Germany's economy was in total disarray.
After that, things then start to look grim for German prospects. It's going to get worse.
