Related Rule
United States of America
Practice Relating to Rule 160. Statutes of Limitation
The US Instructor’s Guide (1985) provides: “There is no statute of limitations on the prosecution of a war crime.” 
United States, Instructor’s Guide – The Law of War, Headquarters Department of the Army, Washington, April 1985, p. 13.
The US Naval Handbook (1995) provides: “There is no statute of limitations on the prosecution of 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), § 6.2.5.3.
The US Naval Handbook (2007) states: “There is no statute of limitations on the prosecution of 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, § 6.2.6.2.
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 (“VAVAO”), 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. With regard to statutes of limitation, the Court stated:
The principle of non-applicability of statutory limitations to certain violations of international law has been recognized in international instruments. The Convention on the Non-Applicability of Statutory Limitations to War Crimes and Crimes Against Humanity provides that “[n]o statutory limitations period shall apply” to war crimes and crimes against humanity, including genocide. Convention on the Non-Applicability of Statutory Limitations to War Crimes and Crimes Against Humanity, Nov. 26, 1968, art. 1, 754 U.N.T.S. 73, 75 (entered into force Nov. 11, 1970); see also Rome Statute of the International Criminal Court (“Rome Statute”), July 17, 1998, arts. 5, 29, U.N. Doc. A/Conf. 183/9 (1998) (entered into force July 1, 2002) (“The crimes within the jurisdiction of the Court [(i.e., genocide, crimes against humanity, war crimes and ‘the crime of aggression’)] shall not be subject to any statute of limitations.”). Although the United States is not a signatory to either the United Nations Convention on the Non-Applicability of Statutory Limitations to War Crimes and Crimes Against Humanity or the Rome Statute, these instruments suggest the need to recognize a rule under customary international law that no statute of limitations should be applied to war crimes and crimes against humanity. 
United States, Eastern States District Court (EDNY), Agent Orange case, Judgment, 28 March 2005, pp. 97–98.
In 1967, during a debate in the Third Committee of the UN General Assembly on the question of the punishment of war criminals and of persons who have committed crimes against humanity, the representative of the United States stated:
Her delegation supported the basic human rights objectives sought through the adoption of a convention on the non-applicability of statutory limitation to the kinds of crimes of which Nazi criminals were prosecuted and convicted at Nürnberg, namely war crimes and crimes against humanity and would co-operate with other delegations which wished to approach the question in a constructive manner. 
United States, Statement before the Third Committee of the UN General Assembly, UN Doc. A/C.3/SR.1517, 16 November 1967, § 9.
In 1977, in reply to a question from the Embassy of France, the US Department of State stated:
It is the view of the United States Government that neither the [1945 London Agreement], with the [1945 IMT Charter (Nuremberg)] annexed, nor [the 1945 Allied Control Council Law No. 10] … contain any provisions setting a time limit for prosecution or punishment. The United States further regards [the 1945 Allied Control Council Law No. 10] as revoking the benefits of any statute of limitation in respect of the period specified; and in light of the absence of any provision to the contrary, the offenses covered in these instruments are considered not to be subject to limitation concerning their prosecution and punishment.
United States Federal law contains no statute of limitations on war crimes and crimes against humanity. 
United States, Department of State, Note addressed to the Embassy of France, 19 May 1977, Department of State File No. P77 0090-522, reprinted in John A. Boyd, Digest of United States Practice in International Law, 1977, US Department of State Publication 8960, Washington, D.C., 1979, p. 927.
In 1991, in a diplomatic note to Iraq, the United States stated:
The Government of the United States reminds the Government of Iraq that under International Law, violations of the Geneva Conventions, the Geneva [Gas] Protocol of 1925, or related International Laws of armed conflict are war crimes, and individuals guilty of such violations may be subject to prosecution at any time, without any statute of limitations. This includes members of the Iraqi armed forces and civilian government officials. 
United States, Department of State, Diplomatic Note to Iraq, Washington, 19 January 1991, annexed to Letter dated 21 January 1991 to the President of the UN Security Council, UN Doc. S/22122, 21 January 1991, Annex I, p. 2.
In another such diplomatic note, the United States reiterated that “Iraqi individuals who are guilty of … war crimes … are personally liable and subject to prosecution at any time”. 
United States, Department of State, Diplomatic Note to Iraq, Washington, 21 January 1991, annexed to Letter dated 22 January 1991 to the President of the UN Security Council, UN Doc. S/22130, 21 January 1991, p. 4.