Conquest of Serbia

Austria Hungary's first target iwas Serbia. In 1914 they attacked Serbia in order to take revenge. They failed. The Central Powers then needed more alliance, so they found Bulgaria who lost Balkan War, and hated Serbia.

Since the history here is not changed until Battle of Verdun, you can refer to wikipedia for this battle.

Result
Serbia was conquered by the joint army of Austria-Hunagry and Bulgaria. They escaped to Albania, but then Albania was captired by Austria-Hungary. They then escaped to sea.