Image courtesy of the National Guard.
Submitted by an author preferring the anonymity of the pen name “Scipio Africanus”:
BLUF: The legal concept of proportionality is not well understood by military officers and is a source of considerable confusion. Military institutions should ensure that legal training articulates the difference between 1) The concept of proportionate response in self-defense, 2) The concept of weighing proportionality of civilian collateral damage in relation to military advantage and 3) The principle of economy of force. Past failure to teach this distinction has resulted in a generation of confused officers that erroneously believe that force must always be employed in proportion to the enemy threat. Proportionate response is only applicable in cases of self-defense, and does not apply when there is zero probability of civilian collateral damage. As long as there is no chance of civilian collateral damage, a commander’s weapon choice is not limited by the law of war.
Last week, I listened as a group of fellow majors discussed their thoughts on the concept of proportionality. It was soon clear that as a group we were deeply confused. We had just received a JAG briefing explaining that proportionality is the process of weighing civilian collateral damage against the relative military advantage of striking a target.
Poor legal training has drilled into many military leaders that proportionality is the concept that only the minimum amount of force should be used to meet the threat and to complete the mission. The confusion between these principles has led to a generation of officers who erroneously believe that it is not in accordance with the laws of war to “kill a fly with a sledgehammer”.
It was only after reading an outstanding 2003 article published in the Marine Corps Gazette, that I finally understood the mystery of proportionality.  There are three separate concepts that are often confused:-
1) Proportionality, which weighs civilian collateral damage with military advantage.
2) Proportionate response which only applies during instances of self-defense, when only the minimum amount of force should be employed to defeat the threat.
3) Economy force which is the principle of war that only the minimum essential combat power should be expended on secondary efforts. 
More than ten years after the publication of the article in the Marine Corps Gazette, proportionality is still explained in confusing terms within official directives. See the below excerpt from the current  CJCSI 3121.01B Standing Rules of Engagement for U.S. forces:-
“(3) Proportionality. The use of force in self-defense should be sufficient to respond decisively tohostile acts or demonstrations of hostile intent. Such use of force may exceed the means and intensity of the hostile act or hostile intent, but the nature, duration and scope of force used should not exceed what is required. The concept of proportionality in self-defense should not be confused with attempts to minimize collateral damage during offensive operations.” 
Yes, we must minimize civilian destruction, but the concept of proportionality states that collateral damage is acceptable if it is outweighed by military advantage. Although the law of war is designed to protect civilians and minimize unnecessary suffering of combatants, the law of war doesn’t prescribe the size of the weapon that we can use to kill the enemy to accomplish the mission.  Therefore, it is perfectly acceptable to use a sledgehammer to kill that pesky little fly that is ruining your AO.
 Gregory G. Gillette, “Proportionality in the Law of War.” Marine Corps Gazette 87, no. 9 (September 2003): 60-62.
 U.S. Department of Defense. JP 3-0: Joint Operations. Washington DC: Joint Chiefs of Staff, 11 August 2011, www.dtic.mil/doctrine/new_pubs/jp3_0.pdf, A-1.
 U.S. Department of Defense. CJCS List of superseded directives, Washington DC: Joint Chiefs of Staff, 7 January 2014, www.dtic.mil/cjcs_directives/support/cjcs/cjcssupcana.pdf
 U.S. Department of Defense, CJCSI 3121.01B Standing Rules of Engagement, Washington DC: Joint Chiefs of Staff, 13 June 2005, posted on Small Wars Journal.
 Gregory G. Gillette, “Proportionality in the Law of War.” Marine Corps Gazette 87, no. 9 (September 2003): 62.