The Ems Dispatch is a unique German decision that gives a free casus belli to 
France against 
North German Federation or 
Prussia.
Historically, the dispatch was used to provoke the French Empire into attacking Prussia/NGF. This led to the Franco-Prussian War, a short war that resulted in Prussian victory and the end of the French Empire. France lost Alsace-Lorraine to Prussia and Prussia eventually forms Germany through this war.
France has to be an Absolute Monarchy instead of a republic and Prussia/NGF has to own Mainz. Next, Prussia/NGF needs 30 prestige and Steel Breech Loaded Artillery researched. Then finally, relations with France needs to be below 0.
- Be prepared, this decision will not give you a reward but a war instead.
If the decision is taken, France will gain cut down to size casus belli against you. This hopefully makes France declare war on you and begin the Franco-Prussian War. The advantage of having a defensive war is, that it is more likely that Prussias/NGFs allies will join, while France's will not. If the war goes well, regain core casus belli can be added without infamy.
Ingame description
Originally sent as an internal memo of King Wilhelm I of Prussia to the Prussian Chancellor Otto von Bismarck, concerning demands made by the French Ambassador during an informal meeting between him and the Monarch at the Spa resort Bad Ems in Prussian Hesse-Nassau, the Ems dispatch was perhaps the most important triggering factor that led to France's declaration of what would later be known as the Franco-Prussian war. By sending the Ems dispatch, we will risk provoking a war with France.