No Plan Survives Contact with the Enemy.
My variant on this is that "No Plan Survives Contact with Reality."
This maxim has served me well.
There is another interesting quote from von Moltke's nephew who was also named Helmut von Moltke. The younger served as Chief of the German General Staff at the beginning of the First World War. The Germans had an elaborate plan for going to war. It involved concentrating all of their forces against the French to quickly defeat them and then those forces would turn and deal with the Russians. This was an elaborate plan that involved railway schedules and was very German in its detail.
The Kaiser was convinced that he could keep the French out of the war and Germany would just have Russia to deal with. The Kaiser called von Moltke in for a meeting and told him that Germany did not need to go to war with France. Just turn the army around and we will go to war with Russia, instead. The response from von Moltke was:
Your Majesty, the movements of millions are not improvised.
The German General Staff had a plan and they were going to stick to it. There was no contingency for going to war with Russia and not France.
The Kaiser replied, "Your uncle would have given me a different answer."
von Moltke the younger was always being measured against his uncle.
Anyway, Germany went to war with both France and Russia, just as they planned it. This plan involved crossing through Belgium, which meant that Britain joined the war. The rest is, as they say, history.