This essay has been focused on the relationship between international law and power. The author starts with doctrine of classical legal thought and negative reaction of the followers of legal realism to this classicism. The realists generally view international law as a result of the interests of powerful states. H. Morgenthau was maybe the first who launched a broad attack on classical legal thought, stressing the decisive relationship between power, state behavior and international law. Realism became the leading position among U.S scholars in international law and the US foreign policy officials. The element of power and a hegemon’s national interest have got nevertheless very soon increasingly dominant position in the US foreign policy. The realists of all schools have many times argued, that international law should not constrain pursuit of the U.S. national interest and foreign policy. International law cannot be detached from politics but on the other side international law cannot be replaced by politics or morality. International law is the body of legal norms which governs the relations of states and other subjects of international law. It is not possible to deny the legally binding character of international law which constrains the freedom of actions of states and other subject. There is the duty of every state to fulfil in good faith its obligations under the generally recognized principles and rules of international law. Art. 103 of the UN Charter stipulates that in the event of a conflict between the obligations of members of the UN at least and their obligations under any other international agreements, their obligations under the Charter shall prevail. Claiming that the UN Charter collective security system has failed gives no justification to the realist conclusion that the use of force is no more subject to the rule of law and that national self-interest prevails. The author is trying to analyse the relationship between the legality and legitimacy in international law. Recourse to unilateral use force on the basis of legitimacy reason without the UNSC authorization has been in fact the claim that any humanitarian intervention is a legitimate exception to the prohibition of the use of force. This article constitutes also an attempt to reveal the basic difference between the rule and the policy approaches in international law. Author deals also with the extensive interpretation of legality and legitimacy of the use of force and in brief with the politics of international law.