How and why is International Law Binding on States?

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[...] A flagrant example could be the case of the Gabčikovo-Nagymaros Project opposing Hungary and Slovakia: the International Court of Justice declared here that both parties had breached the treaty binding them and decided consequently to maintain the original text and obligations of the states, willing to protect the norm of international law. This case shows not only the extent of the binding effect for states, which must strictly follow the duties they signed up for, but also the current phenomenon of instances and organisations attempting to preserve the international legal system when facing domestic resistance or laxism in the application. Not only is international law binding on states-parties through treaties, but to a certain extent it also creates general obligations no matter if the country signed a formal text or not. [...]

[...] Indeed, today the main international norms are treaties, and these texts rely entirely on the countries' will to be bound to be considered valid. Thus, not only non-parties states have no obligation to follow the rules, but some treaties also have very limited effect because they fail to create a sufficient framework, either lacking signatories or not applying to the most concerned countries. For example, while the Ottawa Treaty of 1997 was signed by more than 130 countries seeking to ban the landmines, it was not ratified by the major actors (the largest arms manufacturers: USA, Russia and China) who privileged their domestic interests, giving minimal effect to the restrictions. [...]

[...] Indeed, the existence of customary international law is recognised by a vast majority of actors including the ICJ, who defines it as "evidence of a general practice accepted as law". This source is unwritten, recognized with 2 elements which are state practice and opinio juris (the belief that there is a legal obligation), and is thus considered as being generally binding for all states, without needing their consent to apply. The idea of global compliance with rules also manifests itself with the concept of jus cogens, or "peremptory norms of general international law" . [...]

[...] On another hand, the liberalist view focuses on the nature of the political regime to explain the degree of compliance of different states, with democracy presenting higher chances of respect of international law. Lastly, the constructivists believe that the fulfillment of obligations relies mainly on the will to follow common standards and principles, these understandings acting as a form of moral imperative and making the follow of rules legitimate. No matter the true reason behind each individual compliance with international law, states generally tend to respect global rulings and judgments. Furthermore, although the factor of voluntariness is important, the international legal system also has several mechanisms to formally bind states. [...]

[...] One of the main criticisms of international law is indeed that the institutions, even if capable of law-making, are not able to completely assure its respect due to lack of enforcement mechanisms. The global legal system relies on the idea of equality between sovereign states, without any submission to other bodies except if they agree to do so, thus international organisations depend on the countries' will to function. If a breach is committed, the consequences for the culprit will rely on other states' armed forces (if a military response is envisaged) or economic and commercial policies (if a trade sanction is chosen). [...]