If the United States were attacked by an adversary, NATO members would be obligated under Article Five of the treaty to view it as an attack on themselves — that is unless the attack occurs in Hawaii, Guam, or Puerto Rico.

Article Six of the NATO treaty specifically lays out the geographic limitations of Article Five, which is that Article Five is only applicable if the attack occurs at a location north of the Tropic of Cancer. Hawaii, which became a state in 1959; Guam, which became a U.S. territory in 1898; and Puerto Rico, which became a U.S. territory in 1917, are all south of that latitude.

The 12 original members signed the NATO treaty on April 4, 1949, 75 years ago. One of the major debates the original members had while negotiating the treaty was over whether to include countries’ territories. The U.S. and Canada wanted to exclude all colonial territories, while France wanted to protect its colonies, according to the NATO website.

There is an effort to get Hawaii formally and officially included in Article Five, given its status as a U.S. state.

Earlier this week, a dozen bipartisan senators signed on to a letter to Secretary of State Antony Blinken, emphasizing “the importance of clarifying that members of the North Atlantic Treaty Organization would consider an armed attack against the State of Hawaii to be an attack against all NATO countries.”

Sens. Mazie Hirono (D-HI) and Brian Schatz (D-HI) both signed the letter. Schatz, on Twitter, said the Hawaiian “residents deserve equal protection,” and described the situation as “an outdated technicality.”

The senators argued that getting Hawaii covered under Article Five is imperative given the “strategic importance of the Indo-Pacific — and the need to deter destabilizing regional conflict,” adding, “Hawaii is the center of the U.S. Indo-Pacific strategy, geographically located in the heart of the Pacific Ocean and home to USINDOPACOM headquarters and critical component commands and defense installations.”

The senators requested Blinken answer a series of questions by Sept. 1.

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The U.S. was dragged into World War II after the Japanese military carried out the bombing of Naval Station Pearl Harbor, Hawaii, on Dec. 7, 1941, killing about 2,400 people.

American Samoa, the Northern Mariana Islands, and the U.S. Virgin Islands are also south of the Tropic of Cancer.