Related Rule
United States of America
Practice Relating to Rule 15. The Principle of Precautions in Attack
Section B. Avoidance or minimization of incidental damage
The US Rules of Engagement for the Vietnam War (1971) stated: “While the goal is maximum effectiveness in combat operations, every effort must be made to avoid civilian casualties, minimize the destruction of private property, and conserve diminishing resources.” 
United States, Rules of Engagement for the Employment of Firepower in the Republic of Viet-Nam, US Military Assistance Command Viet-Nam, Directive No. 525-13, May 1971, unclassified contents reprinted in Eleanor C. McDowell, Digest of United States Practice in International Law, 1975, US Department of State Publication 8865, Washington, D.C., 1976, pp. 814–815, § 3(a).
The US Rules of Engagement for Operation Desert Storm (1991) required soldiers to avoid harming civilians and civilian property “unless necessary to save US lives”. 
United States, Desert Storm – Rules of Engagement, Pocket Card, US Central Command, January 1991, reprinted in Operational Law Handbook, International and Operational Law Department, The Judge Advocate General’s School, United States Army, Charlottesville, Virginia, 1995, pp. 8-7 and 8-8, §§ B and G.
The US Naval Handbook (1995) requires naval commanders to “take all reasonable precautions, taking into account military and humanitarian considerations, to keep civilian casualties and damage to the minimum consistent with mission accomplishment and the security of the force”. 
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), § 8.1.2.1.
The US Naval Handbook (2007) states:
Naval commanders must take all reasonable precautions, taking into account military and humanitarian considerations, to keep civilian casualties and damage to the minimum consistent with mission accomplishment and the security of the force. 
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, § 8.3.1.
It is reported that in 1952, during the Korean War, a US General gave the instruction “to attack specific military targets at Pyongyang and to make every effort to avoid needless civilian casualties”. 
Robert F. Futrell, The United States Air Force in Korea 1950–1953, Office of Air Force History, US Air Force, Washington, D.C., Revised edition, 1983, p. 515.
In 1966, in the context of the Vietnam War, the US Department of Defense stated: “All possible care is taken to avoid civilian casualties.” 
United States, Department of Defense, Statement on targeting policy in Vietnam, 26 December 1966, reprinted in Marjorie Whiteman, Digest of International Law, Vol. 10, Department of State Publication 8367, Washington, D.C., 1968, p. 427.
On 30 December 1966, in reply to an inquiry from a member of the US House of Representatives requesting a restatement of US policy on targeting in North Vietnam, a US Deputy Assistant Secretary of Defense wrote: “All reasonable care is taken to avoid civilian casualties.” 
United States, Letter from Deputy Assistant Secretary of Defense Goulding to US Representative Ogden Reid from New York, 30 December 1966, reprinted in Marjorie Whiteman, Digest of International Law, Vol. 10, Department of State Publication 8367, Washington, D.C., 1968, p. 428.
In 1972, the General Counsel of the US Department of Defense stated:
A review of the operating authorities and rules of engagements for all of our forces in Southeast Asia, in air as well as ground and sea operations, by my office reveals that not only are such operations in conformity with this basic rule [that the loss of life and damage to property must not be out of proportion to the military advantage to be gained], but that in addition, extensive constraints are imposed to avoid if at all possible the infliction of casualties on noncombatants and the destruction of property other than that related to the military operations in carrying out military objectives. 
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, pp. 122–124.
In 1986, in the context of US attacks on Libyan targets, the United States stated:
The United States exercised great care in restricting its military response to terrorist-related targets. It took every possible precaution to avoid civilian casualties and to limit collateral damage … In carrying out this action, the United States took every possible precaution to avoid civilian casualties and to limit collateral damage. 
United States, Letter dated 14 April 1986 to the President of the UN Security Council, UN Doc. S/17990, 14 April 1986.
In 1987, the Deputy Legal Adviser of the US Department of State affirmed:
We support the principle that all practicable precautions, taking into account military and humanitarian considerations, be taken in the conduct of military operations to minimize incidental death, injury, and damage to civilians and civilian objects. 
United States, Remarks of Michael J. Matheson, Deputy Legal Adviser, US Department of State, The Sixth Annual American Red Cross-Washington College of Law Conference on International Humanitarian Law: A Workshop on Customary International Law and the 1977 Protocols Additional to the 1949 Geneva Conventions, American University Journal of International Law and Policy, Vol. 2, 1987, pp. 426–427.
In 1991, in response to an ICRC memorandum on the applicability of IHL in the Gulf region, the US Department of the Army stated:
The obligation of distinguishing combatants and military objectives from civilians and civilian objects is a shared responsibility of the attacker, defender, and the civilian population as such. An attacker must exercise reasonable precautions to minimize incidental or collateral injury to the civilian population, consistent with mission accomplishment and allowable risk to the attacking force. 
United States, Message from the Department of the Army to the legal adviser of the US Army forces deployed in the Gulf, 11 January 1991, § 8(E), Report on US Practice, 1997, Chapter 1.6.
In 1991, in a report submitted to the UN Security Council on operations in the Gulf War, the United States stated: “The military actions initiated by the United States and other States co-operating with the Government of Kuwait … are directed strictly at military and strategic targets and every effort has been made to minimize civilian casualties.” 
United States, Letter dated 17 January 1991 to the President of the UN Security Council, UN Doc. S/22090, 17 January 1991, p. 2.
In another such report, the United States stated: “Allied aircraft involved in these attacks are taking every precaution to avoid civilian casualties. These pilots are in fact placing themselves in greater danger in order to minimize collateral damage and civilian casualties.” 
United States, Letter dated 30 January 1991 to the President of the UN Security Council, UN Doc. S/22173, 30 January 1991, p. 1.
In a subsequent report, the United States reiterated: “Coalition forces have taken every precaution to minimize collateral damage to civilian facilities.” 
United States, Letter dated 8 February 1991 to the President of the UN Security Council, UN Doc. S/22216, 13 February 1991, p. 1
In 1991, in a diplomatic note to Iraq concerning operations in the Gulf War, the United States stated: “Hostilities must be conducted in a manner so as to minimize injury to civilians.”  
United States, Department of State, Diplomatic Note to Iraq, Washington, 19 January 1991, annexed to Letter dated 21 January 1991 to the President of the UN Security Council, UN Doc. S/22122, 21 January 1991, p. 2.
In 1992, in its final report to Congress on the conduct of the Gulf War, the US Department of Defense stated:
An attacker must exercise reasonable precautions to minimize incidental or collateral injury to the civilian population or damage to civilian objects, consistent with mission accomplishment and allowable risk to the attacking forces … As correctly stated in Article 51(8) of [the 1977 Additional] Protocol I, a nation confronted with callous actions by its opponent (such as the use of “human shields”) is not released from its obligation to exercise reasonable precaution to minimize collateral injury to the civilian population or damage to civilian objects. This obligation was recognized by Coalition forces in the conduct of their operations … As frequently noted during the conduct of the conflict, exceptional care was devoted to minimize collateral damage to civilian population and property. 
United States, Department of Defense, Final Report to Congress on the Conduct of the Persian Gulf War, 10 April 1992, Appendix O, The Role of the Law of War, ILM, Vol. 31, 1992, pp. 625, 627 and 644.
In 1992, a legal review by the US Department of the Air Force of the legality of extended range anti-armour munition stated that, while legal as such, “care should also be taken [when using such munition] to ensure that the possibility of collateral civilian casualties is minimized, and that it is always used with a neutralizing mechanism”. 
United States, Department of the Air Force, The Judge Advocate General, Legal Review: Extended Range Antiarmor Munition (ERAM), 16 April 1992, § 9.
In 1993, in its report to Congress on the protection of natural and cultural resources during times of war, the US Department of Defense stated:
The obligation to take reasonable measures to minimize damage to natural resources and cultural property is shared by both an attacker and a defender. A number of steps can be taken by an attacker in order to minimize collateral damage to natural resources or cultural property … [During the Gulf War,] the U.S. and its Coalition partners in Desert Storm recognized that they were fighting in the “cradle of civilization” and took extraordinary measures to minimize damage to cultural property … Other steps were taken to minimize collateral damage. Although intelligence collection involves utilization of very scarce resources, these resources were used to look for cultural property in order to properly identify it. Target intelligence officers identified the numerous pieces of cultural property or cultural property sites in Iraq; a “no-strike” target list was prepared, placing known cultural property off limits from attack, as well as some otherwise legitimate targets if attack on the latter might place nearby cultural property at risk of damage. 
United States, Department of Defense, Report to Congress on International Policies and Procedures Regarding the Protection of Natural and Cultural Resources During Times of War, 19 January 1993, pp. 203–205.
In 1998, when announcing the missile attacks against targets in Afghanistan and Sudan, the US President stated that “every possible effort to minimize the loss of innocent life” had been made. The Chairman of the Joint Chiefs of Staff noted that the attacks were carried out at night time in order to minimize the incidental loss of civilian life, and the President’s National Security Adviser stated that the government had verified that no night shift was at work at the chemical plant bombed in Sudan. The Defense Secretary stressed that the possibility of an airborne plume of toxic chemicals from the Sudanese plant had been taken into account in an effort to minimize civilian casualties. 
United States, Presidential Address, 20 August 1998; Press Conference by Secretary of Defense, 20 August 1998, Yearbook of International Humanitarian Law, Vol. 2, 1999, p. 423.
The Report on US Practice states:
The opinio juris of the United States is that measures to minimize civilian casualties and damage must be undertaken to the extent that military necessities permit under the circumstances ruling at the time. The measures might include warnings, care in selecting targets, weapons and methods of attack and, especially against targets in inhabited areas, breaking off attacks that may not be sufficiently accurate. 
Report on US Practice, 1997, Chapter 1.6.