War is the province of chance. In no sphere of human activity is such a margin to be left for this intruder, because none is so much in constant contact with him on all sides. He increases the uncertainty of every circumstance, and deranges the course of events.

From this uncertainty of all intelligence and suppositions, this continual interposition of chance, the actor in War constantly finds things different from his expectations; and this cannot fail to have an influence on his plans, or at least on the presumptions connected with these plans. If this influence is so great as to render the pre-determined plan completely nugatory, then, as a rule, a new one must be substituted in its place; but at the moment the necessary data are often wanting for this, because in the course of action circumstances press for immediate decision, and allow no time to look about for fresh data, often not enough for mature consideration.

But it more often happens that the correction of one premise, and the knowledge of chance events which have arisen, are not sufficient to overthrow our plans completely, but only suffice to produce hesitation. Our knowledge of circumstances has increased, but our uncertainty, instead of having diminished, has only increased. The reason of this is, that we do not gain all our experience at once, but by degrees; thus our determinations continue to be assailed incessantly by fresh experience; and the mind, if we may use the expression, must always be “under arms”.

Think, too, of the great part that is played by the unpredictable in war: think of it now, before you are actually comitted to war. The longer a war lasts, the more things tend to depend on accidents. Neither you nor we can see into them: we have to abide their outcome in the dark. And when people are entering upon a war they do things the wrong way round. Action comes first, and it is only when they have already suffered that they begin to think.

History is littered with good ideas taken too far, which are indistinguishable from bad ideas. The wisdom in having room for error is acknowledging that uncertainty, randomness, and chance — “unknowns” — are an ever-present part of life. The only way to deal with them is by increasing the gap between what you think will happen and what can happen while still leaving you capable of fighting another day.

In war, luck is half in everything.

War itself is not a mere science but a more fickle sort of thing, often subject to fate or chance, being an entirely human enterprise...

Fortune, which has a great deal of power in other matters but especially in war, can bring about great changes in a situation through very slight forces.

We still encounter considerable doses of messiness, accident, fortuitous coupling, and dumb luck.

War involves in its progress such a train of unforeseen and unsupposed circumstances ... that no human wisdom can calculate the end.

...only the element of chance is needed to make war a gamble, and that element is never absent.

Humans sometimes make surprising choices, and human history is full of uncertainties.

War is very uncertain in its results, and often when affairs look most desperate they suddenly assume a more hopeful state.

(Athenian embassy:) Realise, while there is time, the inscrutable nature of war; and how when protracted it generally ends in becoming a mere matter of chance, over which neither of us can have any control, the event being equally unknown and equally hazardous to both.

The misfortune is that in their hurry to go to war, men begin with blows, and when a reverse comes upon them, then have recourse to words.

(Book 1 Chapter 78.1-3)

Fortune leaves always some door open to come at a remedy.

One should always be a little improbable.