Related Rule
Argentina
Practice Relating to Rule 101. The Principle of Legality
Argentina’s Law of War Manual (1969) provides: “Tribunals will only apply legal provisions existing previously to the offence.” It adds that the “legal provisions decreed by the Occupying Power … cannot have a retroactive effect”. 
Argentina, Leyes de Guerra, RC-46-1, Público, II Edición 1969, Ejército Argentino, Edición original aprobado por el Comandante en Jefe del Ejército, 9 May 1967, §§ 5.025 and 5.026.
Argentina’s Law of War Manual (1989) states that the prisoner shall be informed of the offence for which he or she is charged, an offence “which shall constitute a criminal act at the time it was committed”. 
Argentina, Leyes de Guerra, PC-08-01, Público, Edición 1989, Estado Mayor Conjunto de las Fuerzas Armadas, aprobado por Resolución No. 489/89 del Ministerio de Defensa, 23 April 1990, § 3.30.
The manual further states that criminal provisions “cannot have a retroactive effect” and that the alleged offence against the accused “must constitute a criminal act at the moment it was committed”. 
Argentina, Leyes de Guerra, PC-08-01, Público, Edición 1989, Estado Mayor Conjunto de las Fuerzas Armadas, aprobado por Resolución No. 489/89 del Ministerio de Defensa, 23 April 1990, §§ 5.08 and 5.09 (occupied territory); see also § 4.15 (civilians).
The same provision applies in non-international armed conflicts. 
Argentina, Leyes de Guerra, PC-08-01, Público, Edición 1989, Estado Mayor Conjunto de las Fuerzas Armadas, aprobado por Resolución No. 489/89 del Ministerio de Defensa, 23 April 1990, § 7.10.
Argentina’s Law on the Implementation of the 1998 ICC Statute (2006) states: “No provision establishing criminal offences under the Rome Statute [1998 ICC Statute] or this law shall be applied in violation of the principle of legality”. 
Argentina, Law on the Implementation of the 1998 ICC Statute, 2006, Article 13.