Related Rule
United States of America
Practice Relating to Rule 53. Starvation as a Method of Warfare
The Annotated Supplement to the US Naval Handbook (1997) states:
Art. 54(1) [of the 1977 Additional Protocol I] would create a new prohibition on the starvation of civilians as a method of warfare … which the United States believes should be observed and in due course recognized as customary law. 
United States of America, Annotated Supplement to the Commander’s Handbook on the Law of Naval Operations, prepared by the Oceans Law and Policy Department, Center for Naval Warfare Studies, Naval War College, Newport, Rhode Island, November 1997, § 8.1.2, footnote 15.
The Agent Orange case in 2005 involved a class action suit filed on behalf of various Vietnamese nationals and an organization, The Vietnamese Association for Victims of Agent Orange/Dioxin, against Dow Chemical and other US chemical manufacturers, for harms allegedly done to them and their land through the United States’ use of Agent Orange and other herbicides during the Vietnam War from 1965 to 1971 and by the South Vietnamese Government’s subsequent use of such herbicides until 1975. In dismissing the claims, the Court found that, while recognizing the evolution of international law since 1975, the use of herbicides did not violate, at the time they were used, either customary or conventional international law binding on the United States. On the question of whether the destruction of the enemy’s food sources was prohibited, the Court stated:
The United States was seeking to aid the Vietnamese, not wipe them out. And there was no internationally recognized human right that would have required an armed force to refrain from using herbicides to protect its troops and those of its allies. No recognized source of international law that might have applied up to 1975 could have been interpreted as outlawing use of herbicides in the way they were utilized in Vietnam.
Nor, as to destruction of food sources, where this tactic has apparent military advantage, was there a generally accepted prohibitory rule of international law. Investiture of cities to starve the occupants (both military and civilian) into surrender was common. Collection of food and fodder from the country to feed troops and deny it to the enemy troops and civilians supporting those troops was accepted.
In World War I the British by their naval blockade attempted to starve the Germans. See, e.g., ARTHUR HERMAN, TO RULE THE WAVES: HOW THE BRITISH NAVY SHAPED THE MODERN WORLD 493, 513 (2004) (referring to “a long-distance blockade on Germany, in order to ‘strangle the whole national life of the enemy.’”); BENJAMIN A. VALENTINO, FINAL SOLUTIONS: MASS KILLINGS AND GENOCIDE IN THE TWENTIETH CENTURY 85 (2004) (“During the First World War, more than 250,000 people died of starvation and malnutrition when the British blockaded Germany and Austria-Hungary in an effort to starve them into surrender.”). In World War II the Germans attempted to starve the British, and the United States to starve the Japanese, by unrestrained submarine warfare. HERMAN, supra, at 535, 545. Particularly where so much of the enemy force is guerilla in nature and lives off the land, as in the Vietnam War, destruction of crops supporting mobile forces can not be said to have been contrary to tradition up to 1975, even if the international view of its appropriateness may have changed subsequently. 
United States, Eastern States District Court (EDNY), Agent Orange case, Judgment, 28 March 2005, pp. 212–214.
In 1987, the Deputy Legal Adviser of the US Department of State affirmed: “We support the principle that starvation of civilians not be used as a method of warfare.” 
United States, Remarks of Michael J. Matheson, Deputy Legal Adviser, US Department of State, The Sixth Annual American Red Cross-Washington College of Law Conference on International Humanitarian Law: A Workshop on Customary International Law and the 1977 Protocols Additional to the 1949 Geneva Conventions, American University Journal of International Law and Policy, Vol. 2, 1987, p. 426.
In 1987, the Legal Adviser of the US Department of State, referring, inter alia, to the protection of the civilian population against deliberate starvation as contained in the 1977 Additional Protocol II, stated:
For the most part, the obligations contained in Protocol II are no more than a restatement of the rules of conduct with which the United States military forces would almost certainly comply as a matter of national policy, constitutional and legal protections, and common decency. 
United States, Remarks of Judge Abraham Sofaer, Legal Adviser, US Department of State, The Sixth Annual American Red Cross-Washington College of Law Conference on International Humanitarian Law: A Workshop on Customary International Law and the 1977 Protocols Additional to the 1949 Geneva Conventions, American University Journal of International Law and Policy, Vol. 2, 1987, pp. 461 and 462.
In 1991, in response to an ICRC memorandum on the applicability of IHL in the Gulf region, the US Department of the Army stated: “U.S. practice does not involve methods of warfare that have as their intention the starvation of the enemy civilian population.” 
United States, Letter from the Department of the Army to the legal adviser of the US Army forces deployed in the Gulf region, 11 January 1991, § 8(O), Report on US Practice, 1997, Chapter 4.1.
According to the Report on US Practice, it is the opinio juris of the United States that the starvation of civilians as a method of warfare is prohibited. 
Report on US Practice, 1997, Chapter 4.1.
The US Field Manual (1956), in a chapter dealing, inter alia, with siege warfare, states that, in such a situation, “provision is … made in Article 23 [of the 1949 Geneva Convention IV] for the passage … of essential foodstuffs, clothing, and tonics intended for children under 15, expectant mothers, and maternity cases”. 
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, § 44.
The US Naval Handbook (1995) states: “Neutral vessels and aircraft engaged in the carriage of qualifying relief supplies for the civilian population … should be authorized to pass through the blockade cordon.” 
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), § 7.7.3.
The US Naval Handbook (2007) states that “neutral vessels and aircraft engaged in the carriage of qualifying relief supplies for the civilian population … should be authorized to pass through the blockade cordon”. 
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, § 7.7.3.
In 1973, a Deputy Legal Adviser of the US Department of State expressed the hope that
new rules can … be developed to reduce or eliminate the possibility that starvation will result from blockade, perhaps by requiring the passage of food supplies provided only that distribution is made solely to civilians and is supervised by the ICRC or some other appropriate external body. 
United States, Address by George H. Aldrich, Deputy Legal Adviser of the Department of State, 13 April 1973, reprinted in Arthur W. Rovine, Digest of United States Practice in International Law, 1973, Department of State Publication 8756, Washington, D.C., 1974, pp. 503–504.