Following the assassination of Archduke Franz Ferdinand, the Austro-Hungarian Empire passed sanctions and threatened invasion of what nation?

Serbia

Explanation:

After the assassination of the Austro-Hungarian heir, Archduke Franz Ferdinand in Sarajevo led to the series of the diplomatic and military rises. The killing of the royal couple by a Serbian nationalist in Sarajevo led the Austro - Hungarian Empire pass sanctions and threatened invasion in Serbia. All this started the European empire and nations to begin the first World War in 1914 in Europe. The war extended and formed two sides, the Central Powers and Allies.