This article examines the legal architecture of the prohibition on the use of force under the UN Charter and evaluates whether Israel’s strike on Iran can be justified within, or as an evolution of, Article 51’s self-defense framework. While the Charter system traditionally restricts unilateral force to narrow circumstances involving an actual armed attack, the article argues that existential threats, particularly those involving declared intentions of annihilation combined with nuclear capability, may warrant a more flexible interpretation of ‘imminence’. By contrasting Israel’s situation with controversial precedents such as the 2003 Iraq invasion and Russia’s 2022 attack on Ukraine, the article contends that preventive force, though legally fraught, may be justified where a State faces credible, explicit, and long‑standing threats to its survival.