Related Rule
United States of America
Practice Relating to Rule 21. Target Selection
The US Air Force Pamphlet (1976) states: “When a choice is possible between several military objectives for obtaining a similar military advantage, the objective to be selected shall be that which may be expected to cause the least danger to civilian lives and to 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)(c).
On 16 April 1986, in the context of US attacks on Libyan targets, the US President stated that “these targets were carefully chosen, both for their direct linkage to Libyan support of terrorist activities and for the purpose of minimizing collateral damage and injury to innocent civilians”. 
United States, Identical letters dated 16 April 1986 from the US President to the Speaker of the House and the President of the Senate concerning US airstrikes against Libya on 14 April 1986, reprinted in Marian Nash (Leich), Cumulative Digest of United States Practice in International Law, 1981–1988, Department of State Publication 10120, Washington, D.C., 1993–1995, p. 3405.
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 language of Article 57(3) of [the 1977 Additional] Protocol I … is not part of customary law. The provision applies “when a choice is possible …;” it is not mandatory. An attacker may comply with it if it is possible to do so, subject to mission accomplishment and allowable risk, or he may determine that it is impossible to make such a determination. 
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(H), Report on US Practice, 1997, Chapter 1.6.