Related Rule
United States of America
Practice Relating to Rule 67. Inviolability of Parlementaires
The US Field Manual (1956) states:
[A parlementaire] has the right to inviolability, as well as the trumpeter, bugler or drummer, the flag-bearer and the interpreter who may accompany him.
Fire should not be intentionally directed on parlementaires or those accompanying them. If, however, the parlementaires or those near them present themselves during an engagement and are killed or wounded, it furnishes no ground for complaint. It is the duty of the parlementaire to select a propitious moment for displaying his flag, such as during the intervals of active operations, and to avoid the dangerous zones by making a detour. 
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 and 461.
The manual further states: “In addition to the ‘grave breaches’ of the Geneva Conventions of 1949, the following acts are representative of violations of the law of war (‘war crimes’): … firing on the flag of truce.” 
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, § 504(e).
The US Air Force Pamphlet (1976) provides: “In addition to the grave breaches of the Geneva Conventions of 1949, the following acts are representative of situations involving individual criminal responsibility: … deliberate firing on … the flag of truce.” 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 15-3c(3).
The US Instructor’s Guide (1985) states: “In addition to the grave breaches of the Geneva Conventions, the following acts are further examples of war crimes: … firing on the flag of truce”. 
United States, Instructor’s Guide – The Law of War, Headquarters Department of the Army, Washington, April 1985, p. 13.
The US Naval Handbook (1995) stipulates: “Enemy forces displaying a white flag should be permitted an opportunity … to communicate a request for cease-fire or negotiation.” 
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 Handbook adds: “The following acts are representative war crimes: … firing on flags of truce.” 
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), § 6.2.5(11); see also § 8.2.3 (protection of cartel ships).
The US Naval Handbook (2007) states: “Enemy forces displaying a white flag should be permitted an opportunity to … communicate a request for cease-fire or negotiation.” 
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 further states that “firing on flags of truce” is an example of an act that could be considered a war crime. 
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, § 6.2.6.
In 1951, in the 27th Report of the UN Command Operations in Korea to the UN Security Council, the United States reported the following incidents:
On 9 August, General Nam Il, through his Liaison Officer, claimed that the United Nations Command had violated its guarantees by attacking a Communist vehicle plainly marked with white cloth and carrying a white flag. The sole guarantee ever given by United Nations Command Liaison Officer with regard to aircraft refraining from the attack of the Communists delegations’ vehicles was contingent upon their being properly marked and upon prior notification being given of the time and route of their movement. The latter specification had not been complied with and United Nations aircraft did machine gun the truck. The United Nations Command cannot accept the risk of its forces entailed in refraining from attacks on any vehicles observed in rear of the battle zone except those reported by the Communist delegation as being in the service of the delegation. On 14 August, the Communists complained of a like incident. They have been informed again that the United Nations Command provides no immunity for vehicles unless the time and route of movement have been communicated to the United Nations Command. 
United States, 27th Report of the UN Command Operations in Korea to the Security Council, covering the period 1–15 August 1951, annexed to Note dated 15 October 1951 to the UN Secretary-General, UN Doc. S/2377, 16 October 1951, reprinted in Marjorie M. Whiteman, Digest of International Law, Vol. 10, Department of State Publication 8367, Washington, D.C., 1968, p. 398.
On the basis of the US position with regard to the incidents described in the 27th Report of the UN Command Operations in Korea to the UN Security Council in 1951, the Report on US Practice states:
In principle, parlementaires advancing under a white flag should be respected, but in practice advance arrangements should be made to ensure respect for them. While within their own lines, they cannot rely solely on the white flag to protect them or their vehicles from air attack or other indirect fire. 
Report on US Practice, 1997, Chapter 2.2.