Treaties can be compared to contracts: both are means of willing parties assuming obligations among themselves, and a party to either that fails to live up to their obligations can be held liable under International Law. A treaty may also be known as (International) agreement, protocol, Covenant, Convention, exchange of letters, etc. Regardless of the terminology, all of these international agreements under International Law are equally treaties and the rules are the same. 

1) What is Treaty?

     A treaty is an agreement or contract entered into between two or more states whereby they undertake to carry out obligations imposed on each of them. According to Oppenheim, " International Treaties are agreements of a contractual character, between States or Organizations of States creating legal Rights and obligations between the parties"   International treaties are Important Source of International Law.

See...General Principles of Interpretation of International Treaties

2) Formation of treaties:

               There is no specific form for the conclusion of treaties. An oral agreement between the representatives of the States charged with the task of conducting negotiations and empowered to bind their respective countries is sufficient to have a binding effect if it is the intention of the representative to conclude a legally binding transaction. The enormous importance of the issue involved in such agreements however necessities the compliance of formal requirements and reducing the agreements into a document.

See... Various Stages of formation of the treaties

3) Treaty as an Important Source of International Law : 

           The word " International law" is synonyms and equivalent to the words "law of nations ". It was first used by Jeremy Bentham in 1989. Prior it,  International Law was known as the law of nations. Most of the jurists are of the opinion that International Law regulates a relation of states with one another and they, therefore, define the term of International law as a law of nations.
         According to Oppenheim International Treaties are agreements, of a contractual character between States or organizations of States, creating legal rights and obligations between the Parties.  Treaties and Customs are regarded as the exclusive sources of International Law. According to Lawrance, a treaty is an important Source of International Law and an Instrument for imposing the binding obligation.

See in detail... Importance of the Treaties as source of International Law 

4) Vienna Convention on Law of treaties.

       United Nations Conference on the law of treaties held in Vienna from 9 April to 22 May in the year 1969,  at which 110 states were represented adopted the Vienna Convention on the law of treaties. It consists 85 Articles and covers such topics as the Conclusion and entry into force of treaties reservations, application and interpretation and invalidity and termination.

       The opening of the Convention for signature and certification was the culmination of 20 years of work on the subject in the International Law Commission, the general assembly and the conference which held its first session in 1968.  As treaties  now provide most of the legal framework of international relations and as the customary rules governing them where frequently unclear and subject of dispute, the clarification of those rules in a convention is a contribution to the objectives laid down in the United Nations charter "to establish conditions under which justice and respect for the obligations arising from treaties can be maintained."

      The Vienna Convention on the law of treaties (1969) is the first essential element of infrastructure that has been worked out in the enormous task of  Codifying International Law pursuant to article 13 of the United Nations charter. The previous Codification treaties, the four conventions on the law of sea, the Vienna Convention on diplomatic relations; Vienna Convention on consular relations and convention on the reduction of the statelessness, did not despite their intrinsic importance grapple with the fundamentals of the constructing a world legal order system

See also...

Termination of Treaties by Mutual Consent

Grounds of Invalid Treaties

Sources of International law


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