Following archduke Franz Ferdinand's assassination, Austro-Hungarian leaders:

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.

threatened to invade Serbia if serb leaders did not agree to set of strict demands.