On September 23, the US began to bomb targets in Syria. That same day, Samantha Power, the United States Ambassador to the United Nations, sent a letter to Secretary-General Ban Ki Moon notifying him, and the Security Council, that the United States had initiated "necessary and proportionate military actions" in Syria to combat the threat posed by the militant Islamic terrorist groups ISIS (which the letter refers to as ISIL), and Khorasan.
The letter makes two major arguments. First, it justifies the US action against ISIS on the grounds of collective self-defense, asserting that ISIS is attacking Iraq from safe havens in Syria, and that Iraq has asked the United States to lead an international effort to combat that problem.
Second, it justifies the action against Khorasan, which it describes as "al-Qaida elements in Syria," on the basis of individual and collective self-defense: it describes the military action as necessary to "address terrorist threats that they pose to the United States and our partners and allies."
Article 51 of the UN Charter affirms that, in the event of an armed attack, all countries have an "inherent right of individual or collective self-defense." However, any measures taken in self-defense must be reported to the Security Council immediately. It appears that Power's letter was written in order to fulfill that requirement.
Read the whole letter here: