Issues of sovereignty over land are not resolved by the law of the sea; one must look to other sources of international law. Yet while the rules on sovereignty over land are different from the rules for maritime entitlements, there is a close connection between sovereignty and maritime entitlements. Thus, for a State to make claims with respect to maritime zones that are predicated on the existence of an island, that State must have sovereignty over that island. Though sovereignty over many of islands in the world is undisputed, in some instances disputes exist between two or more States. When this happens, each State seeks to prove its sovereign title to the island. There are various ways that a State might be said to have acquired sovereignty over an island, notably: (1) title by discovery of territory that is terra nullius; (2) title by agreement with another State that had sovereignty or a claim to sovereignty; (3) title by State succession; (4) title by military conquest; and (5) title by continuous and peaceful display of sovereignty. Where title over an island exists based on one of these methods and is unchallenged by other States, then it will be regarded as valid regardless of the degree of governmental authority actually exercised over the island. It should be noted that when sovereignty disputes arise among States with respect to an island, arguments advanced by the disputing States often involve an amalgam of these methods of securing title. Further, doctrines of acquiescence, estoppel, abandonment, or tacit agreement may become relevant when considering competing State claims concerning an island, as well as a State’s renunciation of title over an island.

Employing a descriptive-analytical method, this contribution extensively discusses each of these methods for acquiring and proving sovereignty over islands in the mirror of international case law including judicial decisions and arbitral awards. According to the finding of the paper, for many centuries, title over an island was asserted by States based on discovering an island that was terra nullius. Title acquired in this manner may be referred to as “original” title, because (unlike the other forms of title discussed below), it is not based on a “derivative” title secured from another sovereign, such as through cession, prescription, or conquest. This method of securing title is closely associated with the period of colonialism, when imperialist powers often laid claim to territory that in fact was occupied by native peoples. When a State has sought to rely solely on the terra nullius doctrine, its claim has typically failed due to the presence of persons or authorities from another State in the claimed territory. Consequently, the issue of discovery is usually advanced as the starting point for an “inchoate” title, which is then perfected through additional conduct, such as effective occupation or exercise of administrative authority over the island.

Furthermore, a more likely way today to assert title to an island is by reference to an agreement by which one State cedes sovereignty over an island to another State, or otherwise agrees to or acknowledges that other State’s sovereignty over the island. Such an agreement might speak to a broad range of territory that encompasses islands only in part. Alternatively, the agreement might be directed at a specific island or islands. It is worth mentioning that the agreement must be by an authority that is already exercising sovereignty or has a valid claim to sovereignty over the island. Such agreements need not be formal treaties; other forms of tacit agreement or acquiescence may suffice. It should be added that an agreement that a third party shall decide sovereignty is also relevant in determining sovereignty, whether or not that decision is an arbitral award. In some instances, even an unratified agreement might be viewed as probative regarding the facts on the ground. Finally, an agreement between two States might have an effect on a claim of sovereignty by a third State which has been notified of the agreement and does not protest it. In addition, a third way sovereign title to an island might arise is when a predecessor State dissolves or disappears, leaving behind two or more new States, or when a portion of an existing State breaks away, resulting in a new State that claims sovereignty over an island based on the sovereignty of the predecessor State. If the predecessor State held good title to the island on whatever basis (such as discovery or international agreement), then a successor State may be found to have inherited that title by succession. Of course, title by succession requires that there was an uninterrupted transfer of that title; if evidence is lacking in that regard, succession cannot be said to have occurred.

At one time, a State could acquire sovereignty over an island through military conquest. This method of acquiring title, however, is no longer available due to the emergence in international law of a prohibition on the use of force by one State against another. Although sovereignty cannot today be acquired through military conquest, the acquisition of title by this method prior to the emergence of the prohibition on use of force cannot today be challenged. And last but not the least, In the event that title does not exist based on one of the above grounds, a final method for establishing it is through continuous and peaceful display of sovereignty over the island. Two important concepts often arise in this context. First, an international court or tribunal will inquire into whether a State’s title can be inferred from its effective manifestation of State functions over the island (often referred to as effectivités) through acts performed by the said State. Second, when assessing such acts, it is often necessary for a court or tribunal to determine the “critical date” on which the dispute arose. The critical date is important because sovereignty concerning an island may change over time; establishing that a State performed acts à titre de souverain at some time in the past is not sufficient for establishing that the State still had sovereignty at a later time. Thus, the critical date is a point before which all facts relating to the dispute can be said to have occurred. At the same time, the critical date is also the point after which self-serving conduct of the parties can no longer affect the outcome of the dispute.

And Finally, in proving sovereignty based on these methods, States may rely on international law doctrines of acquiescence, estoppel, or tacit agreement, as well as renunciation of title over or abandonment of an island. Furthermore, geographical proximity of the island to an uncontested territory may be considered as a relevant factor in proving sovereignty over islands; however, this element is not such determinative in this regard. Maps can be referred to as well to establish discovery, occupation or display of sovereignty over an island throughout different periods of time, though they must be faced with caution. Generally speaking, it appears that history, geography, politics and law all should be taken into account in the disputes on sovereignty over islands.