I ll go ahead and link a map showing the territorial acquisitions of the Central Powers in the treaty of Brest Litovsk. Germany occupied the Baltic states and Belarus, while they set up a puppet government in the central Ukraine with Austria Hungary occupying the southern half of it.

Austria-Hungary was actually in a serious position, even after the treaty of Brest-Litovsk was signed and the war on the Eastern Front was halted. First, in the newly acquired zones, large numbers of occupation troops had to be stationed. (This meant that there were fewer troops to fight on the other fronts, something which hampered Imperial Germany in particular but Austria-Hungary as well). It was thought they could exploit these territories for food to support the war efforts on the Italian, Western and Serbian fronts, but the territories werent as productive as they hoped to be.

Italy hadnt been knocked out of the war yet. Although they had been defeated by the Germans and Austro-Hungarians back at Caporetto, they recuperated and were eventually back in force, this time with the support of munitions and numerous experienced personnel from Britain, France and America. The Austro Hungarian advance was held in check at the Battles of the Piave River. After that, the Austro Hungarian army was decisively defeated at the Battle of Vittorio Veneto against the Italians and was losing ground rapidly at the Macedonian and Serbian fronts. Its safe to say that the Austro-Hungarian army was facing serious problems even before 1918. After the battle of Vittorio Veneto they were probably in shambles.

All this time there had been increasing nationalist sentiments amongst Austria-Hungary's numerous ethnic minorities.They felt it was their time to delare independence and autonomy. The war was a massive drain on the economy. People were starving, many men were lost in the prior battles against the former Russian Empire and there were shortages everywhere, that lead widescale dissatisfaction for the war effort and riots. The Czechoslovakia region declared independence in 28th October 1918, followed shortly by both the Southern Slavs and the Hungarians.

To sum it up, widespread internal dissent and revolution within their own borders, as well as repeated failures in their war against the Allied Powers meant they were pushed to the brink, and beyond recovery, leading to the official collapse and dissolution of the Empire.