Article 1 of the 1974 General Assembly Resolution on the Definition of Aggression defines aggression as the use of armed force by a State against the sovereignty, territorial integrity, or political independence of another State, or in any other manner inconsistent with the Charter of the United Nations. If the use of military force is in the context of an armed attack as referred to in Article 51 of the UN Charter, the attacked State shall have the right to self-defense. Today, States commonly invoke this right to justify the use of military force, even when unwarranted, and the UN Charter and international customary law have set out specific conditions for its realization. Based on the said Article, self-defense is permissible only when an armed attack has occurred. Furthermore, doctrine requires proportionality of the defensive action to the armed attack and necessity for the realization of lawful self-defense. This research, while examining the key conditions and requirements of self-defense, along with historical examples and judicial practice, analyzes the challenges faced by States in preserving the right to self-defense after the cessation of aggression. This is because, following the end of territorial aggression and the termination of the armed attack, the legitimacy and modality of invoking the right to use military forces in self-defense are subject to doubt. Initial study results indicate that at the time of the aggression's termination, States must carefully evaluate the conditions and provide convincing reasons for the continuation of defense, as the conditions for resorting to self-defense in this framework are complex and dependent on multiple factors.