The conditions for making lawful reservations to treaties

Do reservations diminish the object and purpose of a Treaty?
This question could well be posed by a naturalist. From a naturalist point of view, one could state that reservations do undermine the purpose of a Treaty, for, what is the use of a Treaty, if states are- de facto- allowed to exclude the (ICJ) jurisdiction, especially when it comes to treaties concerning human rights, as is one of the main topical issues, the multilateral Convention on the prevention and punishment of the crime of genocide, Art. IX?

1.1. Positivist approach: efficiency and efficacy
From a positivist approach, however, to some extent, compromises are inevitable in the treaty-making process; the treaty-making process requires efficiency.  Efficiency could not be reached if every party to a treaty has to vote unanimously on a large number of matters- which was, in fact, the case before 1940. It goes to show that negotiations often take decades before a "final" decision is issued.
Another positivist argument on the permissibility of reservations can be found in the efficacy of international rules. States are more likely to adhere to certain rules if they are able to shape the rules, more or less, to their own discretion.

1.2. The USSR case and the ICJ Opinion on Reservations
The conclusion that reservations to treaties are permissible, stems from the 1948 Genocide Convention. Not having recognized the jurisdiction of the International Court of Justice, the USSR proposed reservations to article IX of the Genocide Convention, discarding the jurisdiction of the ICJ to enforce article IX. The advisory opinion holds that reservations to a treaty are permissible, as long as these reservations are compatible with the object and purpose of the treaty. As you'll notice, this conclusion has found its way into the VCLT, particularly into article 19. Although the ICJ aims at creating a balance between interests of states and the purpose of - in case- the Convention- one could say that the advisory opinion works in favour of the party proposing reservations to a treaty. In the end, state sovereignty prevails.

Thus, the question is no longer if reservations to a treaty are permissible, for allowing a state to attach a reservation is a means of imposing obligations on this state, being or expressing its will to become a party to the treaty. The question is hence, to what extent reservations to treaties are permissible.
Whether it concerns the decision-making process or judging an international (internationally wrong acts, for example) case, there will, by  nature, always be ambiguity. There are no "hard and fast rules" in global politics.

Before I am going to discuss what conditions make reservations to treaties permissible, let's explore the subject, "law of treaties", according to the Vienna Convention on Law of Treaties (abbreviation: VCLT).
Note that the VCLT just entered into force on 27 January 1980; therefore, it should be clear that the VCLT is not applicable to cases prior to 1980- if you would like to know which treaties and conventions could be applied to previous cases, look up the International Court of Justice database.

2.1. Law of Treaties
The Law of Treaties is said to be governed by the VCLT and international customary law. Since the Vienna Convention is a mere codification of customary international law, can these two "sources" of law actually be separated? H.L.A. Hart has drawn a distinction between primary and secondary rules of international law, the first type of rules concerning (substantive)  obligations (such as the prohibition of genocide), the second type governing the application and interpretation of primary rules (often mentioned examples of secondary rules include the regimes of state responsibility and the law of treaties). The ILC emphasizes state responsibility for the sake of efficacy.

The scope of the Vienna Convention is presented in article 1, which states that
"The VCLT applies to treaties between states".  Art. 2, para 1 (a) defines "treaties" as:

"International agreements concluded between States, in written form and governed by international law, whether embodied in a single instrument or in two or more related instruments and whatever its particular designation".

In two cases, the SS Lotus case (1927)  and the SS Wimbledon case (1923), the Permanent Court of International Justice  (PCIJ) has given rise to the idea that two principles underlie the law of treaties. One is that states are sovereign, therefore no valid treaty can do without the free consent of states entering into the treaty, i.o. words consensualism is an essential feature; the other principle is that commiting to a treaty imposes obligations on states: the efficacy of a treaty requires that states perform these treaties in good faith. However, the old Roman brocard "pacta sunt servanda" originally applies to civil law.

2.1.2. Ius cogens
Note that peremptory norms are regarded certain overriding principles of international law, which, speaking with I. Brownlie (Principles of International Law), form a "body of ius cogens". No State may derogate from these principles by way of a treaty. Treaties conflicting with a peremptory norm of general international law are void, art. 53 VCLT. The VCLT makes clear that a peremptory norm of international law is "a norm accepted and recognized by the international community of States as a whole, from which no derogation is permitted and which can only be modified by a subsequent norm of general internation law, having the same character".

2.2. Reservations
According to article 2 (1)(d) a "reservation" means a unilateral statement, however phrased or named, made by a State, when signing, ratifying, accepting, approving or acceding to a treaty, whereby it purports to exclude or modify the legal effect of certain provisions of the treaty in their application to that State".

One main rule conducts the formulation of reservations. As article 19 makes clear, a State may formulate a reservation, unless:
a. the formulation is prohibited by the treaty itself;
b. the treaty provides that only specified reservations, not including the reservation in question, may be made;
c. in cases not falling under sub-paragraphs a and b, the reservation is incompatible with the object and purpose of the treaty.

Highly inspired by the Advisory Opinion of  28 May 1951, the last sub-paragraph has led to an enduring discussion on the "compatibility" with the "object and purpose of a treaty", for there is no general rule to define these concepts of a treaty.

As Scheinin puts it: "..one might think that the VCLT cannot provide solutions to reservations problems in relation to human rights treaties because many of its provisions are written to reflect the operation of multilateral treaty between States in issues where States act in their own interest in respect of other States [...] where the treaty does not establish an independent international mechanism for its application and interpretation" (read "Reservations by States under the ICCPR and its optional protocols", in I. Ziemele, "Reservations to human rights treaties and the Vienna Convention Regime").IUs