Loss exchange ratio (LER) is a figure of merit in attrition warfare.
It is usually relevant to a condition or state of war where one side depletes the resources of another through attrition.
Specifically and most often used as a comparator in aerial combat, where it is known as a kill-ratio. For example, during the Korean War, American combat jets had a kill-ratio of 4-1. This means for every four aircraft shot down by an American aircraft, one American plane was shot down by an enemy fighter.
LER has played a significant role in past wars, especially those that have devolved into stalemate and become wars of attrition. For example, the German objective at the Battle of Verdun (1916) during World War I was not the seizure of any strategic objective, but rather to inflict a LER of 2:1 on the French forces and thereby cripple the French army.
During the First Indochina War, Võ Nguyên Giáp, the leader of the Việt Minh, told his French opposite number that “you can kill ten of my men for every one I kill of yours, and at that rate, I will still triumph.” In fact, the LER was approximately 3:1 in favour of the French, and they did indeed withdraw in defeat. A total of 500,000+ Vietnamese were either killed, wounded or captured during the First Indochina War.
It is arguable that the concept of the LER has become relatively less important in modern Western military doctrine, as some military theories posit that it is just as militarily effective to disrupt enemy forces and outmanoeuvre them, thereby reducing their combat effectiveness without necessarily inflicting massive casualties.