In which war was the highest percentage of casualties attributable to friendly fire?

Friendly fire is an attack by a military force on non-enemy, own, allied or neutral, forces while attempting to attack the enemy, either by misidentifying the target as hostile, or due to errors or inaccuracy. Fire not intended to attack the enemy, such as negligent discharge and deliberate firing on one's own troops for disciplinary reasons, is not called friendly fire. Nor is unintentional harm to non-combatants or structures, which is sometimes referred to as collateral damage. Training accidents and bloodless incidents also do not qualify as friendly fire in terms of casualty reporting.

By the 20th and 21st centuries, friendly-fire casualties have likely become a significant percentage of combat injuries and fatalities. Jon Krakauer provides an overview of American casualties during and since the Second World War:

According to the most comprehensive survey of casualties (both fatal and nonfatal), 21 percent of the casualties in World War II were attributable to friendly fire, 39 percent of the casualties in Vietnam, and 52 percent of the casualties in the first Gulf War. Thus far in the ongoing conflicts in Iraq and Afghanistan, casualty rates are 41 percent and 13 percent, respectively.