https://guide-humanitarian-law.org/content/article/3/blockade/

In order for it to be binding to third party States, it must be effective, and its existence must be officially declared along with a starting date, the territorial geographical limits and a time provided for neutral vessels or aircrafts to be able to leave the area (SRM, arts. 93 and 94).

As a method of warfare, a blockade must comply with international humanitarian law (IHL) and customary IHL (CIHL), which are applicable in both situation of international and non-international armed conflict. This means respecting the principles of military necessity, distinction and proportionality, while protecting civilians and civilian objects. (API, arts. 48, 51(2), 51(5)(b), 52(2) and 57(2)(a)(iii); APII; art. 13(2); CIHL rules 1, 7, 11, 14, 15, 17 and 22).

It is clear that under IHL and CIHL, States are obliged to allow the free passage of relief of an exclusively humanitarian and impartial nature that is indispensable to the survival of the civilian population (GCIV, arts. 23 and 59; API, arts. 70-71; APII, art. 18(2) and CIHL rules 53-56). This is regardless of whether the situation is a blockade, siege or an embargo. IHL foresees no exception to this duty based on the argument of imperative military necessity.

A blockade will be deemed unlawful under IHL if it is implemented in a way that denies the civilian population access to essential goods necessary for their survival. Furthermore, any blockade, siege or embargo that is implemented with the purpose of causing starvation of the civilian population is prohibited. (SRM, art. 102; GCIV, arts. 23 and 55, API, art. 54, APII, art. 14 and CIHL rules 53, 55 and 56). Starvation of civilians is a war crime under the International Criminal Court’s (ICC) jurisdiction in the context of an international armed conflict (arts. 8(2)(b)(xxv) of the Rome Statute). Starvation of civilians and obstruction of humanitarian relief are not war crimes under the general article 8 (2)(e) of the Rome Statute applicable to non-international armed conflict. However, in 2019, the Assembly of States Parties to the Rome Statute adopted an amendment to the definition of war crimes applicable in NIAC detailed in article 8(2)(e). The new article (8(2)(e)(xix) prohibits the intentional use of starvation of civilians as a method of warfare by depriving them of objects indispensable to their survival, including the deliberate prevention of relief. This amendment is only applicable to the 16 States that accepted it in August 2024.

For other States, the other prohibitions remain in force and may be prosecuted as crimes against humanity if they meet the conditions of the Rome Statute definition of the intentional infliction of conditions of life, inter alia the deprivation of access to food and medicine, calculated to bring about the destruction of part of a population (art. 7(2)(b) of the Rome Statute).

The blockade of a State’s ports or coastlines by the armed forces of another State’s could also constitute an act of aggression that could lead to prosecution by the ICC under the specific jurisdictional requirements for the crime of aggression set out in the Rome statute of the ICC (art. 8 bis (2)(c)).

There have been many instances in the past where blockades and sieges have limited or deprived access to essential supplies for civilian populations. The chapter on siege presents the jurisprudence of international tribunals regarding the provision of humanitarian assistance in such situations. Overall, only a small number of decisions of international tribunals elaborate on the concept of blockade. On 31 May 2010, Israeli soldiers raided a flotilla of six ships carrying humanitarian aid and attempting to break Israel’s blockade of the Gaza Strip. Despite the fact that the Israeli military operation resulted in the death of 9 persons, the ICC Prosecutor decided not to investigate the case (decision of the Prosecutor on 16 November 2014, confirmed by final decision of the Prosecutor on 29 November 2017). However, the involvement of the ICJ and ICC in the investigation of war crimes and crimes against humanity committed during the war between Israel and Hamas in the Gaza Strip may lead to relevant decisions on blockade in the coming years.