The fog of war is the uncertainty in situational awareness experienced by participants in military operations. The term seeks to capture the uncertainty regarding one's own capability, adversary capability, and adversary intent during an engagement, operation, or campaign. Military forces try to reduce the fog of war through military intelligence and friendly force tracking systems.

The term has become commonly used to define uncertainty mechanics in wargames.

The word "fog" (German: Nebel), but not the exact phrase, in reference to 'uncertainty in war' was introduced by the Prussian military analyst Carl von Clausewitz in his posthumously published book, Vom Kriege (1832), the English translation of which was published as On War (1873):

War is the realm of uncertainty; three quarters of the factors on which action in war is based are wrapped in a fog of greater or lesser uncertainty. A sensitive and discriminating judgment is called for; a skilled intelligence to scent out the truth.

— Carl von Clausewitz

It has been pointed out that von Clausewitz does not use the exact phrase "fog of war", and also uses multiple similar metaphors, such as "twilight" and "moonlight", to describe a 'lack of clarity'.

The first known use of the exact phrase in text dates to 1836 in a poem entitled "The Battle of Bunker Hill" by McDonald Clarke. The poem describes an assault by British forces upon an American redoubt during the 1775 Battle of Bunker Hill:

Will they dare a third attack?
Is a question seen in every eye;
Old Put across the neck and back,
Rides slowly, their vengeance to defy—
Wildy, in that deadly hour,
The Ramparts shove their bolted shower,
While mid the waving fog of war,
Thunders the Yankee’s loud hurrah"