Finally, this line was drawn in the form of Article 6 of the North Atlantic Treaty. Article 6 is the only provision of the agreement that has ever been formally amended. As adopted on 4 April 1949, it is stated as follows: the contracting parties may unanimously invite any other European state in a position to support the principles of this treaty and contribute to the security of the North Atlantic to join. Any state so invited can become a party to the treaty by filing its instrument of membership with the government of the United States of America. The Government of the United States of America informs each of the contracting parties of the filing of any accession instrument. This treaty is ratified and its provisions are implemented by the contracting parties in accordance with their respective constitutional procedures. The ratification instruments are filed as soon as possible with the Government of the United States of America, which informs all other signatories of any filing. The treaty will enter into force between the states that have ratified it as soon as the ratifications of most of the signatories, including those of Belgium, Canada, Luxembourg, the Netherlands, the United Kingdom and the United States, have been tabled and will enter into force for other states on the date of the tabling of their ratifications. 3. This article is supplemented by Article 6, which provides that the mutual defence obligation under Article 5 of the Treaty could therefore be triggered in five geographical scenarios. First, the obligation could be made by attacks on the territory of one or more signatories, provided that these territories are located in Europe or North America. Overseas territories, with a thesis on the North Atlantic islands, were excluded.
Secondly, attacks were taken on an exceptional basis against the Algerian departments of France. Thirdly, attacks on the occupying forces of any party in Europe could also trigger mutual assistance, even if these forces were attacked outside their own territory. Fourth, the North Atlantic islands were covered north of the Tropic of Cancer. This means that the obligation of mutual defence extends to Greenland, but not to Hawaii. Finally, attacks on ships or aircraft in this area of the North Atlantic have also been covered by the mutual assistance guarantee. Finally, it is important to note that the geographical conditions set out in Article 6 of the North Atlantic Treaty apply only to the article 5 mutual defence obligation, not to another provision of the agreement. Theodore C. Achille, one of the original architects of the treaty, noted that “the authors had no idea that [Article 6] should prevent collective planning, maneuvering or operations south of the Tropic of Cancer in the Atlantic Ocean or any other area important to the safety of the parties.” In other words, collective measures that are not taken under Article 5 of the treaty can be described as “out-of-area”, but it is not outside the borders or ultraviruses for NATO.
The following twelve states signed the treaty and thus became founding members of NATO. The following heads of state and government signed the agreement, as plenipotentiary of their country, on 4 April 1949 in Washington, D.C. During the drafting of Article 5 in the late 1940s, there was a consensus on the principle of mutual assistance, but there were fundamental differences on how to implement this obligation. European participants wanted to ensure that the United States automatically came to the aid if one of the signatories was attacked; the United States did not want to make such a commitment and obtained that this was reflected in the text of Article 5.