The Fog of War refers to the uncertainty that can exist in military operations. It can be used when an adversaries strategies, enemy capabilities, or even the ultimate goal of the enemy is not completely known.
The best and clearest example of this concept is tactical in nature. A commander may not have accurate or sufficient intelligence to ascertain his enemies’ numbers, positions, formation or intent. In such a situation, the commander might comment to his superiors that, “the fog of war is too thick to move at this time.”
Relating this to the Cold War is quite simple. There have been numerous examples of the U.S. and U.S.S.R. having to deal with the fog of war when dealing with each other. During the Cuban Missile Crisis, the U.S. was not aware that Cuba’s missile batteries were indeed operational. On this case the fog of war obscured the true capability of the Cubans.
Another example might be how the U.S. totally underestimated the fighting capabilities of the Vietnamese people during the Vietnam War. The fog of war totally obscured the ability of the U.S. to judge it’s opponents strength and will to fight.