What legitimizes the US' repeated military intervention in Iraq?
Obama's latest intervention is in line with the violent tradition of his predecessors who have been acting in utter disregard for the UN Charter's prohibition of the use of force (Article 2(4)). Under the UN Charter the main power to mandate the use of armed force lies with the UN Security Council. It is the main UN body and the only one able to issue binding and decisive resolutions. Whether President Obama has the exclusive executive authority to allow military action under the domestic legal order is irrelevant for our legal assessment under international law. Obama does not even claim to act for national security reasons, instead he is relying on the need "to prevent a potential act of genocide". He thereby implies to act according to a rule under international law by invoking the need for a 'humanitarian intervention' and apparently does not rely on any domestic legal basis. The question therefore is, whether (a) there is a rule in international law that allows for the unilateral use of military force against another sovereign state in order to prevent an act of genocide and whether (b) this was the case in nothern Iraq given the facts on the ground.
Obama's justification - double standards
President Obama in his speech on August 7th declared that he had authorized targeted airstrikes as "a humanitarian effort to help save thousands of Iraqi civilians who are trapped on a mountain without food and water and facing almost certain death" and argues that this happened "at the request of the Iraqi government". Needless to say, though, that Obama's alleged care for Yezidis once again makes plain the apparent double standards of western governments. The Obama narrative of "Yezidi women, men and children from the area of Sinjar" who "have fled for their lives" and "are now hiding high up on the mountain, with little but the clothes on their backs", without food and water his remarks that "People are starving. And children are dying of thirst" would seem heartbreaking, human and attentive, would not the very same person at the very same time justify, support and even finance the murdering of approximately 2.000 palestinian civilians within 4 weeks, among them up to 400 children, several women, elderly and innocent civilians. A quarter million people have become internally displaced people due to Israeli aggression and are now living in crowded UN refugee shelters, without a sewage system, adequate shelter or even healthcare. These people too, are slowly starving to death, due to a merciless U.S./Israeli/E.U.-embargo against a civilian population. The people of Gaza are under collective punishment by the most powerful world governments for having chosen their favoured leaders in 2006. Where is his compassion, his care and deep concern when it comes to oppressed Palestinians, when it comes to the murder of muslims by U.S.-sponsored mercenaries all over the Middle East? Is it really only the fact that it is Muslims dying and not Christian Yezidi people? How inhumane must the world be to distinguish between valuable and less valuable lives? Human beings have dignity regardless of their race, colour, religion or sex. Human rights are universal irrespective of geographic locations. This is what Islam has taught the world more than 1400 years ago and was has been practiced by muslim leaders throughout the ages but this fact has not yet reached western governments apart from the solemn vows made merely on paper some 60 years ago.
No 'intervention on invitation'
President Obama claims that "we have a mandate to help, in this case, a request from the Iraqi government". He justifies the use of military force with what is known as an "intervention on invitation". An intervention on invitation is understood as the military intervention in an internal armed conflict at the request of the government of the State concerned. In the past, the U.S. and the Soviet Union have repeatedly justified interventions with an alleged invitation by the respected government. This has been the case for example during the U.S. intervention in the Dominican Republic in 1965 or the Soviet's aggression in Afghanistan in 1979. So, while there may be some state practice where states invoked the "intervention on invitation" this state practice can in no way be considered consistent and uniform and above that, it certainly lacks opinio iuris. It can therefore not be considered to have ripened into a customary international law principle, as the vast majority of states have objected to this state practice. Even if in some cases it was not the principle per se that was objected, but rather the authenticity of the request of the respective government, the legality of an "intervention on invitation" in this case still remains dubious. As the U.N. General Assembly stated in its Report of the Special Committee on the Problem of Hungary: ‘The act of calling in the forces of a foreign State for the repression of internal disturbances is an act of so serious a character as to justify the expectation that no uncertainty should be allowed to exist regarding the actual presentation of such a request by a duly constituted Government’.This shows that the expressed request of a government must be clear, unequivocal, attained without pressure or threat and represent the will of its people. These requirements can certainly never be met in cases where so-called "puppet governments" or "vassal regimes" have been installed by the intervening power.
No unilateral 'humanitarian intervention'
Humanitarian interventions have been mandated by the UN Security Council mandate in the past in case of grave human rights violations, for example 1991 against Kurdish people in Iraq. However in other cases of mass genocide and grave human rights violations like in Darfour or in Myanmar in 2012, the Security Council remained inactive. In any case, however, there is no right to a unilateral humanitarian intervention without UN mandate by one state against another if it believes that human rights are in danger. This would be a clear violation of Article 2 (4) UN Charter. The humanitarian intervention by NATO/US forces in Kosovo in 1999 is widely considered to be a breach of international law, as there was no mandate of the UN Security Council. It would furthermore be abusive to use this precedent as state practice in order to construct a legal justification under international law as two Security member states (Russia and China) had objected the intervention. There is therefore no uniform and even less any consistent state practice in favor of an exception from the prohibition of the use of force in form of a unilateral humanitarian intervention.
No collective self-defence
Some commentators have even referred to right to self-defence in Article 51 UN Charter. Accordingly, the U.S. and Iraq are jointly defending themselves against an armed attack. However this view disregards the fact that the 'attack' comes from within Iraqi territory. Moreover, it neglects the requirement of an attack by a foreign state. This is not the case when non-state actors like the ISIS terrorists commit terror attacks. Lastly, Article 51 UN Charter states that the UN Security Council must be notified as soon as possible. Self-defence can only be invoked to react to an imminent attack where no other means would work. In this case the US could have evacuated their staff from Erbil first and notified the UN Security Council. However, as the US has major interests in this conflict and as the ISIS is part of their agenda, they feel no need in adhering to international law.
It turns out that Obama's "targeted airstrikes" in Iraq are not justified under international law. He can neither invoke the right to self-defence, nor is there any rule under customary international law allowing the use of force outside the scope of the UN Charter.
Discussing the legality of the US military intervention by conducting targeted airstrikes actually misses the root cause problem and is too short-sightened. The conflict in Iraq did not just start last week when the US started bombarding ISIL positions in northern Iraq. In fact it has its roots in the failed attempts to enforce regime change in Syria. The U.S. and its european allies have been arming terrorist entities and are paying for the barbaric mercenaries in Syria. The ISIL militia have been trained and equipped by the U.S. The U.S. created the problem and now pretend to be the saviours of Iraq. It is an irony that the military power that has been the root cause for Iraqi sectarian violence over the past decades now pretends to be its saving power. Under international law, the US is legally responsible for the ISIS atrocities. The arming, training and sending of armed groups which carry out acts of armed force against another state across an internaitonal border constitutes an act of aggression according to Article 3 (g) Definition of Aggression. The US has already unlawfully intervened in the Middle East by sending and training mercenaries. The finding of the ICJ in its Nicaragua Decision that "the support given by the United States ... to the military and paramilitary activities of the contras in Nicaragua, by financial support, training, supply of weapons, intelligence and logistic support, constitutes a clear breach of the principle of non-intervention" is not least true for the current US activities in Iraq. They are part of the problem, not the solution.
 UNGA ‘Report of the Special Committee on the Problem of Hungary’ GAOR 11th Session Supp 18 para. 266
 UNGA RES 3314 (XXIX)
 Nicaragua v. United states of America, Merits, Judgment, ICJ Rep. 1986, p. 14 (p. 124, para. 242).