Related Rule
United States of America
Practice relating to Rule 66. Non-Hostile Contacts between the Parties to the Conflict
The US Field Manual (1956) states: “Absolute good faith with the enemy must be observed as a rule of conduct.” 
United States, Field Manual 27-10, The Law of Land Warfare, US Department of the Army, 18 July 1956, as modified by Change No. 1, 15 July 1976, § 49.
The manual also states:
452. One belligerent may communicate with another directly by radio, through parlementaires, or in a conference, and indirectly through a Protecting Power, a third State other than a Protecting Power, or the International Committee of the Red Cross. …
453. It is absolutely essential in all nonhostile relations that the most scrupulous good faith shall be observed by both parties, and that no advantage not intended to be given by the adversary shall be taken.
458. In current practice, radio messages to the enemy and messages dropped by aircraft are becoming increasingly important as a prelude to conversations between representatives of belligerent forces.
469. In its narrower sense, a cartel is an agreement entered into by belligerents for the exchange of prisoners of war. In its broader sense, it is any convention concluded between belligerents for the purpose of arranging or regulating certain kinds of nonhostile intercourse otherwise prohibited by reason of the existence of the war. Both parties to a cartel are in honor bound to observe its provisions with the most scrupulous care, but it is voidable by either party upon definite proof that it has been intentionally violated in an important particular by the other party. 
United States, Field Manual 27-10, The Law of Land Warfare, US Department of the Army, 18 July 1956, as modified by Change No. 1, 15 July 1976, §§ 452, 453, 458 and 469.
The US Naval Handbook (2007) states: “The [1949] Geneva Conventions recognize the special status of the ICRC and have assigned specific tasks for it to perform, including … serv[ing] as a neutral intermediary between belligerents.” 
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, § 6.2.2.
The US Uniform Code of Military Justice (1950) punishes “any person … who communicates or corresponds with or holds any intercourse with the enemy, either directly or indirectly”. 
United States, Uniform Code of Military Justice, 1950, Article 104(2).
According to a memorandum of a legal adviser of the US Department of State in 1975, the president, as commander-in-chief of the armed forces, has the constitutional authority to conclude armistices and other agreements relating to the military security of the United States. 
United States, Legal Adviser to the Department of State, Memorandum of Law on the authority of the US President to enter into international agreements pursuant to his independent constitutional powers, 31 October 1975, reprinted in Eleanor C. McDowell, Digest of United States Practice in International Law, 1975, Department of State Publication 8865, Washington, D.C., 1976, pp. 314-315.
The Report on US Practice states:
The need to seek express authority to negotiate an agreement with the enemy … has been reinforced by the erosion, since the end of World War II, of distinctions between political agreements, such as peace treaties, and purely military agreements, such as truces and armistices … [The Air Force Pamphlet] noted that the practice of concluding peace treaties had become rare, and that armistices had often become functional substitutes for peace treaties. The term “cease fire” was increasingly used for agreements that would once have been designated armistices.
Modern combat conditions may also make it more difficult to communicate directly with an enemy armed force.
US commanders have little inherent authority to negotiate with the enemy, and unauthorized communications with the enemy may be a military offense. The practice of the United States no longer recognizes any clear category of agreements as purely military without political overtones. 
Report on US Practice, 1997, Chapter 2.2. (The report notes that general armistices often include political provisions, and therefore require high-level approval.)
The US Field Manual (1956) notes:
In the past, the normal means of initiating negotiations between belligerents has been the display of a white flag …
The white flag, when used by troops, indicates a desire to communicate with the enemy. The hoisting of a white flag has no other signification in international law. It may indicate that the party hoisting it desires to open communication with a view to an armistice or a surrender. If hoisted in action by an individual soldier or a small party, it may signify merely the surrender of that soldier or party. It is essential, therefore, to determine with reasonable certainty that the flag is shown by actual authority of the enemy commander before basing important action upon that assumption. 
United States, Field Manual 27-10, The Law of Land Warfare, US Department of the Army, 18 July 1956, as modified by Change No. 1, 15 July 1976, § 458.
The US Air Force Pamphlet (1976) states:
The white flag has traditionally indicated a desire to communicate with the enemy and may indicate more particularly, depending upon the situation, a willingness to surrender. It raises expectations that the particular struggle is at an end or close to an end since the only proper use of the flag of truce or white flag is to communicate to the enemy a desire to negotiate. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 8-6a.
The US Naval Handbook (1995) states: “Customary international law recognizes the white flag as symbolizing a request to cease-fire, negotiate, or surrender.” 
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), § 11.9.5.
The US Naval Handbook (2007) states: “Customary international law recognizes the white flag as symbolizing a request to cease-fire, negotiate, or surrender.” 
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, § 8.5.1.5.
The Handbook also states: “Once an enemy warship has clearly indicated a readiness to surrender, such as by … hoisting a white flag … the attack must be discontinued.” 
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, § 8.6.1.
The US Field Manual (1956) provides that parlementaires are “agents employed by commanders to go in person within the enemy lines for the purpose of communicating or negotiating openly and directly with the enemy commander”. 
United States, Field Manual 27-10, The Law of Land Warfare, US Department of the Army, 18 July 1956, as modified by Change No. 1, 15 July 1976, § 459.
The manual states: “A person is regarded as a parlementaire who has been authorized by one of the belligerents to enter into communication with the other and who advances bearing a white flag.” 
United States, Field Manual 27-10, The Law of Land Warfare, US Department of the Army, 18 July 1956, as modified by Change No. 1, 15 July 1976, § 460.
Moreover, the manual states: “Parlementaires must be duly authorized in a written instrument signed by the commander of the forces.” 
United States, Field Manual 27-10, The Law of Land Warfare, US Department of the Army, 18 July 1956, as modified by Change No. 1, 15 July 1976, § 462.
The US Field Manual (1956) provides: “The commander to whom a parlementaire is sent is not in all cases obliged to receive him.” 
United States, Field Manual 27-10, The Law of Land Warfare, US Department of the Army, 18 July 1956, as modified by Change No. 1, 15 July 1976, § 463.
The manual adds:
The present rule is that a belligerent may not declare beforehand, even for a specified period – except in case of reprisal for abuses of the flag of truce – that he will not receive parlementaires. An unnecessary repetition of visits need not be allowed. 
United States, Field Manual 27-10, The Law of Land Warfare, US Department of the Army, 18 July 1956, as modified by Change No. 1, 15 July 1976, § 464.