This was asked before here: http://www.reddit.com/r/AskHistorians/comments/1jpo5e/what_are_the_oldest_international_treaties_still/

I'd like to add a few things though. First, a violation of a treaty is just that: a violation of treaty law. It does not render the treaty null and void, and relieve the party of their obligations under the treaty, unless the treaty specifically says so.

Anyway, as far as individual treaties go, either the Treaty of Windsor (1386), or the treaty between bishops to establish joint ownership over Andorra (1278.

As far as paper treaties go, these are some of the oldest, where there are two clear states who inherited the legal obligations of their predecessors and have the treaties on the books, and can be referenced in a modern court of law.

If you are willing to bend your definition, however, there may be much older treaties still in effect, albeit indirectly. In 1900, the U.S. Supreme Court, ruling on the Paquete Habana (175 U.S. 677) invoked ancient practice, as well as treaties dating back to the 15th and 14th centuries: from Damrosh and Murphy, International Law, Cases and Materials, 6th ed.: "In 1403 and 1406, Henry IV of England issued orders protecting fisherman of foreign states....this practiced, based on prior agreement with the French King for reciprocal treatment, was followed in a treaty made October 2, 1521 between the Emperor Charles V and Francis I of France." (emphasis mine).

This prior agreement, which could constitute an informal treaty, has effectively been honored since. Much of modern maritime law, defined through UNCLOS, derives from various treaties and customs dating back to antiquity. Hugo Grotius' Mare Liberum expanded on some of the ideas, but they were practiced and reflected in treaties between Greek city states.

Other than maritime law (i.e. the right of any nation to prosecute piracy, freedom of navigation on the high seas), there are other aspects of international law that have become customary (or codified into treaty law) that derive from very ancient treaties and practice. These include the Vienna Conventions of treatment of diplomats, which formally codified ancient practice, though it was only signed in 1961- but in preparation of the treaty, practices and treaties from antiquity were invoked.

Alien tort law can be seen similarly- there is a universal principle that States are bound to protect the property of foreign aliens resident in their territories, and that those who are wronged have the right to sue for reparations. This goes back to Hammurabi and Biblical law, though the ability of States to sue one another for the losses of their nationals in a formal court of law is much more recent- however, treaties establishing protections for the property of other States are old as dirt.

Other older treaties can aren't necessarily on the books, but still "in effect" might include older treaties of surrender- i.e. Nizhny Novgorod recognizing Moscow's supremacy.

Though, if you're looking only for treaties that are still on the books and directly invokable in a court of law, with the original signatories who have acknowledged their obligations under the treaty, then Treaty of Windsor (or maybe the Andorran agreements between the French and Spanish) are your best bet. However, other treaties are indirectly still in effect, though they are now simply part of the body of customary international law or have formed the basis for subsequent treaty law.

The same question, with almost the same title, was asked last year: https://www.reddit.com/r/AskHistorians/comments/1jpo5e/what_are_the_oldest_international_treaties_still/

Exactly, it depends on the definition. I just found it relevant to mention.

Regarding the UK, hos does Cromwell fit in? Was it still somehow a constitutional monarchy under him?