There are many reasons that render the "Treaty of Guarantee" invalid including the UN Charter as a result of Cyprus’ 1960 entry into the UN, but in this article we will take a look at what international law has to say regarding the termination of treaties themselves…
The link being used is...
http://www.worldtradelaw.net/misc/viennaconvention.pdf
...and the relevant section begins from the end of page 18 of the PDF.
Vienna Convention on the Law of Treaties
Article 60: Termination or suspension of the operation of a treaty as a consequence of its breach
1. A material breach of a bilateral treaty by one of the parties entitles the other to invoke the breach as a ground for terminating the treaty or suspending its operation in whole or in part.
2. A material breach of a multilateral treaty by one of the parties entitles:
(a) the other parties by unanimous agreement to suspend the operation of the treaty in whole or in
part or to terminate it either:
(i) in the relations between themselves and the defaulting State, or
Vienna Convention on the Law of Treaties
(ii) as between all the parties;
(b) a party specially affected by the breach to invoke it as a ground for suspending the operation
of the treaty in whole or in part in the relations between itself and the defaulting State;
(c) any party other than the defaulting State to invoke the breach as a ground for suspending the
operation of the treaty in whole or in part with respect to itself if the treaty is of such a character
that a material breach of its provisions by one party radically changes the position of every party
with respect to the further performance of its obligations under the treaty.
3. A material breach of a treaty, for the purposes of this article, consists in:
(a) a repudiation of the treaty not sanctioned by the present Convention; or
(b) the violation of a provision essential to the accomplishment of the object or purpose of the treaty.
4. The foregoing paragraphs are without prejudice to any provision in the treaty applicable in the event of a breach.
5. Paragraphs 1 to 3 do not apply to provisions relating to the protection of the human person contained in treaties of a humanitarian character, in particular to provisions prohibiting any form of reprisals against
persons protected by such treaties.
In July 1974, all three guarantor powers of the Republic of Cyprus breached the 1959 Treaty of Guarantee in the following manner…
Greece, overthrew the legitimate government of Cyprus via a military coup violating Cyprus’ government and constitution.
Turkey militarily invaded and occupied a portion of Cyprus violating Cyprus’ territorial integrity and legitimate government.
Britain, having prior knowledge of the schemes of Greece and Turkey failed to take any form of measures to protect the Republic Cyprus.
In accordance with the Vienna Convention on the Law of Treaties all three guarantor powers were in breech of the 1959 Treaty of Guarantee rendering it invalid.
Regards, GR.