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United States of America
Practice Relating to Rule 29. Medical Transports
Section A. Respect for and protection of medical transports
The US Field Manual (1956) restates Article 35 of the 1949 Geneva Convention I and Article 21 of the 1949 Geneva Convention IV. 
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, §§ 236 and 260.
The US Air Force Commander’s Handbook (1980) provides that ambulances … “should not be deliberately attacked, fired upon, or unnecessarily prevented from performing their medical duties”. 
United States, Air Force Pamphlet 110-34, Commander’s Handbook on the Law of Armed Conflict, Judge Advocate General, US Department of the Air Force, 25 July 1980, § 3-2.
The Handbook further stresses that medical transports lose their special immunity if they are used to commit “acts harmful to the enemy outside their humanitarian functions”. In this respect, the manual gives the example of “firing at the enemy from an ambulance”. 
United States, Air Force Pamphlet 110-34, Commander’s Handbook on the Law of Armed Conflict, Judge Advocate General, US Department of the Air Force, 25 July 1980, § 3-2(d).
The US Naval Handbook (1995) states: “medical vehicles … may not be deliberately bombarded. Belligerents are required to ensure that such medical facilities are, as far as possible, situated in such manner that attacks against military targets in the vicinity do not imperil their safety.” 
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), § 8.5.1.4.
The manual qualifies “deliberate attack upon … medical vehicles” as a war crime. 
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.
The US Naval Handbook (2007) states that “medical vehicles … may not be deliberately bombarded. Belligerents are required to ensure that such medical facilities are, as far as possible, situated in such a manner that attacks against military targets in the vicinity do not imperil their safety.” 
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, § 8.9.1.4.
The Handbook also states that examples of war crimes that could be considered as grave breaches of the 1949 Geneva Conventions include: “Deliberate attacks upon … medical vehicles.” 
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(7).
Upon signature of the 1977 Additional Protocols I and II, the United States declared:
It is the understanding of the United States of America that the terms used in Part III of [the 1977 Additional Protocol II] which are the same as the terms defined in Article 8 [of the 1977 Additional Protocol I] shall so far as relevant be construed in the same sense as those definitions. 
United States, Declaration made upon signature of the 1977 Additional Protocols I and II, 12 December 1977, § B.
In 1987, in submitting the 1977 Additional Protocol II to the US Senate for advice and consent to ratification, the US President expressed the view that the obligations in the Protocol were “no more than a restatement of the rules of conduct with which US military forces would almost certainly comply as a matter of national policy, constitutional and legal protections, and common decency”. 
United States, Message from the US President transmitting the 1977 Additional Protocol II to the US Senate for advice and consent to ratification, Treaty Doc. 100-2, 29 January 1987, Comment on Article 10.