The Left Bank
The Left Bank is a decision available to France. It is available if Prussia exists and owns the to provinces Trier (In Rheinland) and Cologne (In Nordrhein). Prussia owns both provinces in the beginning of the game. Otherwise it requires that France has researched Ideological Thought. It cannot be taken if Prussia has formed North German Federation or Germany, so it must be used early in the game.
It reduces the relation with all states in Germany by 100, gives 4 infamy and 11 prestige and gives an acquire state casus belli to France against Prussia. The casus belli only lasts for 1 month, so war has to be declared immidiately.
It is part of the chain of decisions surrounding the Rhine crisis. It leads to the Prussian decisions Die Wacht am Rhein and Niederwald Denkmal. The two decision will remove 7 infamy from Germany, but will not affect France directly, so one should not avoid taking the decision in fear of those two decisions.
The AI will always take the decision, unless infamy is below 0.5.