Self-defence in international law in the context of "Nicaragua v. United States"

International law recognises a right to collective self-defence. This allows a state to come to the aid of another state that has come under attack by a foreign power, in an exception to the prohibition on the use of force in international relations. Collective self-defence is enumerated as an inherent right in the Chapter VII, Article 51 of the Charter of the United Nations.

In geopolitics, an invasion typically refers to a military offensive in which a polity sends combatants, usually in large numbers, to forcefully enter the territory of another polity, with either side possibly being supported by one or more allies. While strategic goals for an invasion can be numerous and complex in nature, the foremost tactical objective normally involves militarily occupying part or all of the invaded polity's territory. Today, if a polity conducts an invasion without having been attacked by their opponent beforehand, it is widely considered to constitute an international crime and condemned as an act of aggression.

Historically, invasions have variously been associated with conquest and annexation, self-defence (if the invader was attacked first), liberation of the invaded polity's people (or of territory that had been occupied by it), or the establishment or re-establishment of control or authority over a territory. Other common motives include forcing the partition of a polity, toppling or altering the established government of a polity or gaining concessions from said government, or intervening to support a belligerent in a polity's civil war.

The right of self-defense is the right for people as individuals to commit a crime, violent or non-violent, for the purpose of defending their own life (self-defense) and property, or to defend the lives of others, in certain circumstances. For example, while reckless driving is usually against the law, it can be justified if it was done to avoid a collision. The right, when it applies to the defense of another, is also called alter ego defense, defense of others, defense of a third person. Nations and states also have a right to self-defense in relation to their existence and independence.

In criminal law, if a defendant commits a crime because of a threat of deadly or grievous harm, or a reasonable perception of such harm, the defendant is said to have a "perfect self-defense" justification. If a defendant commits a crime because of such a perception, and the perception is not reasonable, the defendant may have "imperfect self-defense" as an excuse.

The Application of the Convention on the Prevention and Punishment of the Crime of Genocide in the Gaza Strip (South Africa v. Israel) is an ongoing case that was brought before the International Court of Justice on 29 December 2023 by South Africa regarding Israel's conduct in the Gaza Strip during the Gaza war, that resulted in a humanitarian crisis and mass killings.

South Africa alleged that Israel had committed and was committing genocide in Gaza, contravening the Genocide Convention, including what South Africa described as Israel's 75-year apartheid, 56-year occupation, and 16-year blockade of the Strip. South Africa requested that the ICJ indicate provisional measures of protection, including the immediate suspension of Israel's operations. Israel characterized South Africa's charges as "baseless", accusing the country of "functioning as the legal arm" of Hamas. Israel said that it was conducting a war of self-defense in accordance with international law following the Hamas-led attack on its territory on 7 October 2023.