Is international law really law?
i.         The lack of institutions.
The functions of government can be differentiate into the legislative, executive and judicial to ensure that these functions are exercised by different individuals to avoid tyranny. In the context of International Law, it is not possible to locate a permanent supreme legislative body.
a)      The General Assembly of the United Nations is not such a body and was never intended to be so. The Security Council has power by resolution to regulate the conduct of dispute between states but these powers are limited to specific threats to peace and can only be made within the specific terms of the Charter.
b)      However, even in those limited resolution, it does not have the state organs to implement its resolution. In contrast, in an efficient and well organised state, a legislature will not only be able to pass a law but also be capable of raising the revenue through taxation to provide resources to implement that law.
c)      In the context of the judicial branch, an international court only dates from 1920 and since it does not possess compulsory jurisdiction, even a dispute that is capable of legal analysis may not be referred to the court. Examples; the question of the precise boarder between Iraq and Kuwait. Further, there is no obligation upon States to appear before the International Court of Justice unless the parties have consented in advance.
ii.       International law is determined by the self-interest of individual states.
a)      It is argued that a state may act if its vital interests are threatened but not otherwise. Examples; critic is well placed on the conduct the United States in organising the bombing of Lybia in 1986 or liberating Kuwait in 1990-1991, while showing considerable reluctant to involve itself in the disintegration of Yugoslavia.
b)      Treaties are the closest analogy to legislation in international law however treaties are more akin to contract than legislation; a source of obligation rather than law. If a treaty is viewed as a bargain concluded between parties then it is apparent that parties are more concerned with their own self interest rather than with the general good characteristic of legislation.
The preposition that treaties are consensual obligations gives rise to the following result:
                                                                                      I.            States are not bound unless they consent to them.
                                                                                   II.            Government may be in a position to pick and choose parts of a treaty to obey.
                                                                                 III.            States are empowered to opt out of the process at any time.
iii.      Ubi societas, ibi jus (‘If there is society, law will be there,’ by Cicero).
No society has operated successfully for any length of time without legal rules to balance the competing interest in that society. Law is thus both a social fact and a social necessity.
a)      In the modern world of over 150 autonomous states, law is needed to regulate and promote a degree of cooperation between these states. In the modern world where cooperation is the norm, it is claimed that law is simply a social necessity.
b)      Sovereignty was linked to territory. A ruler of a state was only sovereign within that territory. Such a premise naturally gave rise to questions as to the status of the ruler outside his own territory so that where sovereignty ended the International law began. Therefore, it is argued that International law grew up because of the territorial limitations on sovereignty.
a)      John Westlake argued that every society had a legal system and that international law existed if one could demonstrate that international society existed. With such a large number of international organisations, there can be no doubt that international society existed.   
iv.     International law appears devoid of proper means of enforcement.
a)      People believe that State have little respect for international law and have no incentive to comply with it in the absence of world government. This belief springs firstly from the common misconception that international law is broken with impunity.
b)      The fundamental characteristic of international law is that its function is to regulate the relation between states (entities known to be sovereign and which assert their full independence of any legal order). This raises the problem of how these states which affirm their sovereignty can be subject to international law. If one postulate at the outset that there is no higher authority than the state, how can the norm of international law be produced for and applied by such a sovereign state?   

     Reference: Various sources from International Law textbooks.