Соответствующая норма
Practice Relating to Rule 145. Reprisals
Section B. Reasonable notice and measure of last resort
Croatia’s LOAC Compendium (1991) states that a condition for a reprisal is that it is a “last resort” and that “prior warning” be given. 
Croatia, Compendium “Law of Armed Conflicts”, Republic of Croatia, Ministry of Defence, 1991, p. 19.