Related Rule
United States of America
Practice Relating to Rule 139. Respect for International Humanitarian Law
Section B. Orders and instructions to ensure respect for international humanitarian law
The US Air Force Pamphlet (1976) emphasizes that: “The US … ensures observance and enforcement through a variety of national means including … military regulations [and] rules of engagement.” 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 15-2(e).
The US Manual on Detainee Operations (2008) states: “JFCs [joint force commanders] will ensure that all detainees are treated … IAW [in accordance with] … the law of war”. 
United States, Manual on Detainee Operations, Joint Chiefs of Staff, 30 May 2008, p. III-11.
The manual further states:
When U.S. forces conduct detainee operations, they must possess the text of the applicable [1949] Geneva Conventions … Because the Armed Forces of the United States comply with the law of war as a matter of DOD [Department of Defense] policy during all operations, this requirement is applicable as a matter of policy to all detention operations. JFCs have the overall responsibility to develop, implement, monitor, and, when necessary, refine standards, policies, and SOPs [standard operating procedures] for detainee operations that are consistent with obligations imposed by law and applicable policy. Pursuant to this obligation, JFCs are responsible to ensure the effective routine review of detention operations. 
United States, Manual on Detainee Operations, Joint Chiefs of Staff, 30 May 2008, p. III-14.
The manual quotes a statement by the US President to the UN made on the International Day in Support of Victims of Torture, 26 June 2004:
The United States … remains steadfastly committed to upholding the [1949] Geneva Conventions, which have been the bedrock of protection in armed conflict for more than 50 years. These Conventions provide important protections designed to reduce human suffering in armed conflict. We expect other nations to treat our service members and civilians in accordance with the Geneva Conventions. Our Armed Forces are committed to complying with them. 
United States, Manual on Detainee Operations, Joint Chiefs of Staff, 30 May 2008, p. I-1.
In 1972, the General Counsel of the US Department of Defense considered that:
Rules of engagement are directives issued by competent military authority which delineate the circumstances and limitations under which United States Forces will initiate and/or continue combat engagement with the enemy.
These rules are the subject of constant review and command emphasis. They are changed from time to time to conform to changing situations and the demands of military necessity. One critical and unchanging factor is their conformity to existing international law as reflected in the Hague Conventions of 1907 and the Geneva Conventions of 1949, as well as with the principles of customary international law of which UNGA Resolution 2444 (XXIII) is deemed to be a correct restatement. 
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. 124.