Die Wacht am Rhein is a minor decision for 
Prussia, 
North German Federation, and 
Germany.
During the 1840 Rhine Crisis, in which, the French minister Adolphe Thiers progress the French claim on Upper and Middle Rhine. The member states of the German Confederation feared French aggression and the reimplementation of French Rhineland. Max Schneckenburger, a merchant, wrote a poem called ‘Die Wacht am Rhein’ to calm the German masses.
France has to enact The Left Bank decision in order to make this decision appear. Once the decision appears, Prussia needs to have below -50 relations with France. The Left Bank gives 100 relation with all German states, so it is very likely, that France and Prussia already have a relation of -50 once that decision is taken.
Once enacted, One gains 5 prestige and -3 infamy. There's no threat in using this decision, so it is advised to use it when one has over 3 infamy.
It leads to the Niederwald Denkmal decision, which however requires that Germany is formed.
Ingame description
In 1840, France's then prime minister Adolphe Thiers had claimed that the river Rhine constituted the natural eastern border of the French nation, sparking what would later become known as the Rhine Crisis, as this was perceived in Germany as a threat to Prussian possessions on the western bank of the river. In response to these events, the poet and merchant Max Schneckenburger wrote a patriotic poem calling on the 'Fatherland to remain calm', for, as the poem says, 'fast and true stands the watch at the Rhine', implying that the German nation would defend the river against any and all (French) aggression. The poem was put to music in 1854, and became extremely popular in Germany during the Franco-Prussian war of 1870-71.