One of the most important features of the international arena is the fact that it is, in political science terms, anarchic. This does not mean it is complete anarchy, but it does mean that there is no overarching authority to maintain peace or establish and enforce rules. At times, one powerful nation may play that role, as the United States and other nations before it have. That is a convention that only works from time to time and is not the rule.
The field of international relations might be considered metaphorically to be more like the open plains of the savanna, where all the animals must work out a tentative peace in order to survive. Whether they are predators or prey, friends or competitors, all the animals have times when they are vulnerable and weak, and all have needs that must be met. In the absence of rules or an authority figure, they must work out between themselves how to get along without being in constant danger or conflict. This same dynamic describes the condition of anarchy as it applies to international relations. Forms of cooperation are necessary, even between enemies and competitors.
Laws work when they are enforced. In order to enforce them, an authority with the exclusive right to use physical force must be established. This is one of the important aspects of legitimate governance. The people, in essence, agree to surrender authority of violence to a just force. Such arrangements are not possible in an anarchic environment in which there is no single authority.
Liberalism and realism are two major theoretical viewpoints on international relations. Liberalism rests upon the belief that greater cooperation between nations, specifically through the establishment of international organizations and governing bodies, will lead to a more peaceful and prosperous world. Realism on the other hand, asserts that in this environment, nations are bound to look after their own interests first. It posits that they will do so to the greatest extent they can without jeopardizing their relationships with others.
In essence, one point of view claims that following the laws, even if it is somewhat voluntary, will always be in a state's interest for the peace and prosperity it brings to the world. The other is more pragmatic and points out that nations are not likely to behave in this lawful way no matter what. Rather, they are likely to diverge from such laws when it is clearly in their interest to do so. Furthermore, the consequences for such rule breaking are not likely to be overly severe. A study of international relations is likely to reveal multiple cases that fit one or the other of these points of view.