A League of Nations mandate refers to a legal status for certain territories transferred from the control of one country to another following World War I, or the legal instruments that contained the terms for administering the territory on behalf of the League. These were of the nature of a treaty or convention and contained minority rights clauses that provided for the right of petition and adjudication by the International Court. The mandate system was established under Article 22 of the Covenant of the League of Nations, entered into on 28 June 1919. With the dissolution of the League of Nations after World War II, it was stipulated at the Yalta Conference that the remaining Mandates should be placed under the Trusteeship of the United Nations, subject to future discussions and formal agreements. Most of the remaining mandates of the League of Nations (with the exception of South-West Africa) eventually became United Nations Trust Territories.
All the territories subject to League of Nations mandates were previously controlled by states defeated in World War I, principally Imperial Germany and the Ottoman Empire. The mandates were fundamentally different from the protectorates in that the Mandatory power undertook obligations to the inhabitants of the territory and to the League of Nations.
The process of establishing the mandates consisted of two phases:
The divestiture of Germany's overseas territories was accomplished in the Treaty of Versailles of 1919, and the territories were allotted among the Allied Powers on May 7, 1919. Ottoman territorial claims were first addressed in the Treaty of Sèvres of 1920 and finalized in the Treaty of Lausanne of 1923. The Turkish territories were allotted among the Allied Powers in the Conference of Sanremo of 1920.
Peace treaties have played an important role in the formation of the modern law of nations. Many rules that govern the relations between states have been introduced and codified in the terms of peace treaties.  The first twenty-six articles of the Versailles Treaty of 28 June 1919 contained the Covenant of the League of Nations. It contained the international machinery for the enforcement of the terms of the treaty. Article 22 established a system of Mandates to administer former colonies and territories.
Article 22 was written two months before the signing of the peace treaty, before it was known what "communities", "peoples", or "territories" were related to sub-paragraphs 4, 5, and 6. The treaty was signed, and the peace conference had been adjourned, before a formal decision was made. The mandates were arrangements guaranteed by, or arising out of the general treaty which stipulated that mandates were to be exercised on behalf of the League.
The treaty contained no provision for the mandates to be allocated on the basis of decisions taken by four members of the League acting in the name of the so-called "Principal Allied and Associated Powers". The decisions taken at the conferences of the Council of Four were not made on the basis of consultation or League unanimity as stipulated by the Covenant. As a result, the actions of the conferees were viewed as having no legitimacy.
In testimony before the Senate Committee on Foreign Relations a former US State Department official who had been a member of the American Commission at Paris, testified that England and France had simply gone ahead and arranged the world to suit themselves. He pointed out that the League of Nations could do nothing to alter their arrangements, since the League could only act by unanimous consent of its members - including England and France.
United States Secretary of State Robert Lansing, was a member of the American Commission to Negotiate Peace at Paris in 1919. He explained that the system of mandates was a device created by the Great Powers to conceal their division of the spoils of war under the color of international law. If the former German and Ottoman territories had been ceded to the victorious powers directly, their economic value would have been credited to offset the Allies' claims for war reparations. Article 243 of the treaty instructed the Reparations Commission that non-mandate areas of the Saar Valley and Alsace-Lorraine were to be reckoned as credits to Germany in respect of its reparation obligations. 
Under the plan of the US Constitution the Congress was delegated the power to declare or define the Law of Nations in cases where its terms might be vague or indefinite. The US Senate refused to ratify the Covenant of the League of Nations. The legal issues surrounding the rule by force and the lack of self-determination under the system of mandates were cited by the Senators who withheld their consent.  The US government subsequently entered into individual treaties to secure legal rights for its citizens, to protect property rights and businesses interests in the mandates, and to preclude the mandatory administration from altering the terms of the mandates without prior US approval.
The United States filed a formal protest because the preamble of the mandates indicated to the League that they had been approved by the Principal Allied and Associated Powers, when, in fact, that was not the case.
The Official Journal of the League of Nations, dated June 1922, contained an interview with Lord Balfour (UK) in which he explained that the League's authority was strictly limited. The article related that the 'Mandates were not the creation of the League, and they could not in substance be altered by the League. The League's duties were confined to seeing that the specific and detailed terms of the mandates were in accordance with the decisions taken by the Allied and Associated Powers, and that in carrying out these mandates the Mandatory Powers should be under the supervision--not under the control--of the League.'
The exact level of control by the Mandatory power over each mandate was decided on an individual basis by the League of Nations. However, in every case the Mandatory power was forbidden to construct fortifications or raise an army within the territory of the mandate and was required to present an annual report on the territory to the League of Nations.
Despite this, mandates were generally seen as de facto colonies of the empires of the victor nations.
The mandates were divided into three distinct groups based upon the level of development each population had achieved at that time.
The first group or Class A mandates were areas formerly controlled by the Ottoman Empire that were deemed to "... have reached a stage of development where their existence as independent nations can be provisionally recognized subject to the rendering of administrative advice and assistance by a Mandatory until such time as they are able to stand alone. The wishes of these communities must be a principal consideration in the selection of the Mandatory." The Class A mandates were:
The second group or Class B mandates were all former Schutzgebiete (German territories) in the Sub-Saharan regions of West and Central Africa, which were deemed to require a greater level of control by the mandatory power: "...the Mandatory must be responsible for the administration of the territory under conditions which will guarantee freedom of conscience and religion." The mandatory power was forbidden to construct military or naval bases within the mandates. The Class B mandates were :
and two former German territories, each split in a British and a French League of Nations mandated territory, according to earlier military occupation zones:
A final group, the Class C mandates, including South-West Africa and certain of the South Pacific Islands, were considered to be "best administered under the laws of the mandatory as integral portions of its territory"
The Class C mandates were former German possessions:
According to the Council of the League of Nations, meeting of August 1920 : "draft mandates adopted by the Allied and Associated Powers would not be definitive until they had been considered and approved by the League ... the legal title held by the mandatory Power must be a double one: one conferred by the Principal Powers and the other conferred by the League of Nations,"
Three steps were required to establish a Mandate under international law:(1) The Principal Allied and Associated Powers confer a mandate on one of their number or on a third power; (2) the principal powers officially notify the council of the League of Nations that a certain power has been appointed mandatory for such a certain defined territory; and (3) the council of the League of Nations takes official cognisance of the appointment of the mandatory power and informs the latter that it [the council] considers it as invested with the mandate, and at the same time notifies it of the terms of the mandate, after assertaining whether they are in conformance with the provisions of the covenant." 
After the United Nations was founded in 1945 and the League of Nations was disbanded, all but one of the mandated territories that remained under the control of the mandatory power became United Nations trust territories, a roughly equivalent status. In each case, the colonial power that held the mandate on each territory became the administering power of the trusteeship, except that Japan, which had been defeated in World War II, lost its mandate over the South Pacific islands, which became a "strategic trust territory" known as the Trust Territory of the Pacific Islands under United States administration. The sole exception to the transformation of League of Nations mandates into UN trusteeships was that South Africa refused to place South-West Africa under trusteeship. Instead, South Africa proposed that it be allowed to annex South-West Africa, a proposal rejected by the UN General Assembly. The International Court of Justice held that South Africa continued to have international obligations under the mandate for South-West Africa. The territory finally attained independence in 1990 as Namibia, after a long guerrilla war of independence against the apartheid regime.
Nearly all the former League of Nations mandates had become sovereign states by 1990, including those which had become UN Trust territories except some successor entities of the gradually dismembered Trust Territory of the Pacific Islands (formerly Japan's South Pacific Trust Mandate) - notably the Northern Mariana Islands becoming a United States Commonwealth (still administered by a Governor, without their own Head of State, which remains the US President) - while remnant Micronesia and the Marshall islands, the heirs of the last territories of the Trust, attained on 22 December 1990 final independence (the UN Security Council ratified termination of trusteeship, effectively dissolved on 10 July 1987), and the Republic of Palau (split-off from the Federated States of Micronesia) became the last to get its independence effectively on 1 October 1994).