Helmuth, Graf von Moltke (the Elder):

No battle plan survives contact with the enemy.

Seneca (the Younger):

Vetus proverbium est gladiatorem in harena capere consilium; aliquid adversarii vultus, aliquid manus mota, aliquid ipsa inclinatio corporis intuentem monet. Quid fieri soleat, quid oporteat, in universum et mandari potest et scribi; tale consilium non tantum absentibus, etiam posteris datur: illud alterum, quando fieri debeat aut quemadmodum, ex longinquo nemo suadebit, cum rebus ipsis deliberandum est.

There is an old adage about gladiators, — that they plan their fight in the ring; as they intently watch, something in the adversary’s glance, some movement of his hand, even some slight bending of his body, gives a warning. We can formulate general rules and commit them to writing, as to what is usually done, or ought to be done; such advice may be given, not only to our absent friends, but also to succeeding generations. In regard, however, to that second question, — when or how your plan is to be carried out, — no one will advise at long range; we must take counsel in the presence of the actual situation.

Epistulae Morales 22.1-2, tr. Richard C. Gummere, Loeb Classical Library, 1917