Argument: Nootka Sound Convention does not apply to UK in Falklands
Some points about the Nootka Sound Convention. Article VI" It is further agreed with respect to the eastern and western coasts of South America and the islands adjacent, that the respective subjects shall not form in the future any establishment on the parts of the coast situated to the south of the parts of the same coast and of the islands adjacent already occupied by Spain; it being understood that the said respective subjects shall retain the liberty of landing on the coasts and islands so situated for objects connected with their fishery and of erecting thereon huts and other temporary structures serving only those objects."
1) It is debatable that it applies to the Falklands. It refers to adjacent islands. The Falklands at 300 nautical miles from Argentina are not adjacent to Argentina.
2) It was suspended in 1795 due to war between the two countries. It may or may not have been renewed in 1814 after the war.
3) It's a reciprocal treaty. Both countries, Spain as well as Great Britain (the respective subjects), were forbidden to form establishments on the coasts mentioned. Spain, by forming settlements late 18th -early 19th century in what is now San Clemente del Tuyú (directly south of the Banda Oriental -now Uruguay), was in breach of the Convention
4) If it does apply to the Falklands, Argentina, by establishing a settlement on the Falklands in 1826 (subjects of any other power),rendered article 6 null and void as per the secret article:
Since by article 6 of the present convention it has been stipulated, respecting the eastern and western coasts of South America, that the respective subjects shall not in the future form any establishment on the parts of these coasts situated to the south of the parts of the said coasts actually occupied by Spain, it is agreed and declared by the present article that this stipulation shall remain in force only so long as no establishment shall have been formed by the subjects of any other power on the coasts in question. This secret article shall have the same force as if it were inserted in the convention.
(Argentine web pages on Nootka and the Falklands never mention the secret article)
5) New states do not inherit treaties without the consent of other signatories to those treaties.
6) Argentina did not inherit the Falklands so neither did she inherit any treaty Spain may have signed with any country regarding the Falklands.
"The Argentine Seizure Of The Malvinas (Falkland) Islands", History and Diplomacy. Global Security. 1987 - "The British insist that mutual agreements made between Spain and England during the Nootka Sound Convention of 1790 did not affect existing claims to sovereignty. The agreement stipulated that in the future neither party should establish any settlement on the eastern or western coasts of South America or on adjacent islands to the south of those already held by Spain."
Extended arguments to the contrary
Some point against the extended argument above:
1. Nootka's applicability to the Falklands only became debateable in the latter half of the 20th century, when Argentina used it to assert its own sovereignty claims over the islands. Before then, it was widely held on both sides of the Atlantic that Nootka applied to the Falklands.
2. The treaty was suspended in 1795 when Spain declared war against Britain. The treaty was reinstated in full after the war, according to the Treaty of Amity and Alliance between Great Britain and Spain, article 1, signed the 5th of July of 1814, it being an official document emitted by the British Foreign Office.
3. This was indeed a reciprocal treaty. Britain broke the treaty when British forces invaded the Falklands in 1833. Tuyu was so named in 1580 by Hernando Arias de Saavedra. The area denominated "San Clemente" was first identified later in 1744 by Thomas Falkner during a land survey. The Rosas family had a hacienda; there was no formal Spanish declaration of foundation of any city, in any way or by any officer of the Spanish Crown, such that could be identified, for example, as an event comparable to the founding of Buenos Aires by Juan de Garay. Thus, the first infraction upon Nootka, was in 1833, by the British, at Falklands. It should be noted that the British twice attempted to invade Buenos Aires (both times without success) prior to HMS Clio's arrival to the islands, but at the time the treaty was still suspended, so it doesn't count.
4. The secret article only mentions "the parts of these coasts situated to the south of the parts of said coasts". The secret article DOES NOT MENTION "the islands adjacent". The secret article only mentions establishment of any other power ON THE COASTS IN QUESTION. Thus, any settlements on the islands by Argentina subsequent to its independence would NOT automatically invalidate Nootka.
5. New states do not inherit treaties without consent, true. Newly-formed independent states, however, do inherit territories formerly under the control of their predeceding dependent area, prior to their independence. In the case of Argentina, the predeceding dependent area was the Spanish Viceroyalty of the River Plate, and the islands fell under its jurisdictional control. The principle cited is "Uti Possidetis Juris" and it is a well known principle of international law.
6. Argentina inherited the Falklands, without having to inherit the treaty that Spain signed, because of the fact that at the time of Argentina's independence in 1816, Nootka was in full effect, with Spain exercising full and uninterrupted sovereignty over the islands. Concordantly, the position of the British government in 1920 held that all British claims to the islands prior to 1774 were entirely null and void after 1790.