Assassination of the French ministers plenipotentiary

Between late 1797 and April 1799, a diplomatic congress was held in Rastadt (Bade-Wurtemberg), involving the French Republic, the Holy Roman Empire and German princes. Its aim was to negotiate peace and determine territorial compensation following the French annexation of the left bank of the Rhine. This conference ended not only in failure: it is one of the very few diplomatic gatherings in history in which ambassadors lost their lives.
On the evening of 28 April 1799, the three French diplomats (ie. ministers plenipotentiary) left Rastadt in a carriage with their wives. Lurking in the woods, a troop of Austrian (or Hungarian) hussars stopped them and slaughtered two of the French representatives with bladed weapons. The third, seriously wounded, feigned death and then managed to flee into the woods.
The immense outrage felt in France at this act of violence helped to reignite the war effort against the Second Coalition.