Loss Exchange Ratio

Loss-Exchange Ratio (LER) is a military term that calculates the comparative casualties suffered by each combatant from a battle, engagement or extended conflict. For example, at the Battle of Gettysburg (1863) during the American Civil War, the Union forces suffered approximately 23,000 casualties against 22,000 for the Confederacy — a loss-exchange ratio of roughly 1:1. By way of contrast, at the Battle of Omdurman (1898), the British/Egyptian lost around 482 men, while the Mahdist Sudanese lost about 27,700 — a LER of 57:1.

Contemporary use

The concept of LER has become less important in modern military doctrine as it is now considered much more important to disrupt enemy forces and outmaneuver them, thereby reducing their combat effectiveness without necessarily inflicting massive casualties. However, it 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 an LER of 2:1 on the French forces and thereby cripple the French army.

In asymmetrical warfare

LER has also been highly relevant in asymmetrical conflicts — for example, in the Vietnam War, where despite overwhelming technological superiority, the French and then the Americans were unable to defeat the North Vietnamese forces. Vo Nguyen Giap, the leader of the Viet 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 favor of the French, and they did indeed withdraw in defeat.