Franco-Prussian War
On the French side, planning after the disaster at Wissembourg had become essential. General Le Bœuf, flushed with anger, was intent upon going on the offensive over the Saar and countering their loss. However, planning for the next encounter was more based upon the reality of unfolding events rather than emotion or pride, as Intendant General Wolff told him and his staff that supply beyond the Saar would be impossible. Therefore, the armies of France would take up a defensive position that would protect against every possible attack point, but also left the armies unable to support each other.

Q: Which disaster made French planning supremely essential?
the disaster at Wissembourg