Related Rule
United States of America
Practice Relating to Rule 15. The Principle of Precautions in Attack
Section A. Constant care to spare the civilian population, civilians and civilian objects
The US Air Force Pamphlet (1976) states: “In conducting military operations, constant care must be taken to spare the civilian population, civilians, and civilian objects.” 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 5-3(c)(1)(a).
The US Naval Handbook (1995) states: “All reasonable precautions must be taken to ensure that only military objectives are targeted so that civilians and civilian objects are spared as much as possible from the ravages of war.” 
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-2.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Transportation, US Coast Guard, October 1995 (formerly NWP 9 (Rev. A)/FMFM 1-10, October 1989), § 8.1.
The US Naval Handbook (2007) states: “The law of targeting, therefore, requires that all reasonable precautions must be taken to ensure that only military objectives are targeted so that noncombatants, civilians, and civilian objects are spared as much as possible from the ravages of war.” 
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-12.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Homeland Security, US Coast Guard, July 2007, § 8.1.
In 1972, the General Counsel of the US Department of Defense stated that the United States regarded the principle contained in UN General Assembly Resolution 2444 (XXIII) of 1968 that “a distinction must be made at all times between persons taking part in the hostilities and members of the civilian population to the effect that the civilians be spared as much as possible … as declaratory of existing customary international law”. 
United States, Letter from J. Fred Buzhardt, General Counsel of the Department of Defense, to Senator Edward Kennedy, Chairman of the Subcommittee on Refugees of the Committee on the Judiciary, 22 September 1972, AJIL, Vol. 67, 1973, p. 122.
According to the Report on US Practice, it is the opinio juris of the United States that a “distinction must be made between persons taking part in the hostilities and members of the civilian population to the effect that the civilians be spared as much as possible”. 
Report on US Practice, 1997, Chapter 1.4.
While the core challenges in the protection of civilians identified in the previous reports of the Secretary-General still need our sustained attention, the new report also identifies several protection policy priorities that need to be explored. In particular the following “emerging” issues would benefit from our attention, and the Group of Friends stands ready to act as a platform to advance them. …
… [O]n the issue of lethal autonomous weapons systems (LAWS), the Group is of the view that further discussions are needed and it welcomes the fact that the issue will be examined in Geneva in May 2014, in the framework of the CCW [Convention on Certain Conventional Weapons]. The Group hopes that such discussions will also examine the issue with due consideration to the protection of civilians as part of a comprehensive debate including legal, military operational, technological and ethical perspectives. In time discussion should focus on the relevance of such systems to the protection of civilians, in particular in the context of IHL and with regard to the principles of distinction, precaution and proportionality. 
Uruguay, Statement by the permanent representative of Switzerland during a UN Security Council open debate on the protection of civilians in armed conflict made on behalf of the Group of Friends on the Protection of Civilians in Armed Conflict, namely Australia, Austria, Belgium, Brazil, Canada, France, Germany, Italy, Japan, Liechtenstein, Norway, Portugal, Switzerland, the United Kingdom and Uruguay, 12 February 2014, p. 2.