As with all things, Murphy's Law is operative! Field Marshall Helmuth Karl Bernhard Graf von Moltke, Chief of the Prussian General Staff, defined this concept quite well in his "On Strategy"....

“The material and moral consequences of every major battle are so far reaching that they usually bring about a completely altered situation, a new basis for the adoption of new measures. One cannot be at all sure that any operational plan will survive the first encounter with the main body of the enemy. Only a layman could suppose that the development of a campaign represents the strict application of a prior concept that has been worked out in every detail and followed through to the very end.

Certainly the commander in chief will keep his great objective continuously in mind, undisturbed by the vicissitudes of events. But the path on which he hopes to reach it can never be firmly established in advance. Throughout the campaign he must make a series of decisions on the basis of situations that cannot be foreseen. The successive acts of war are thus not premeditated designs, but on the contrary are spontaneous acts guided by military measures. Everything depends on penetrating the uncertainty of veiled situations to evaluate the facts, to clarify the unknown, to make decisions rapidly, and then carry them out with strength and consistency.”