“The argument for not including Hawaii is simply that it’s not part of North America,” Santoro says.
The exception is spelled out in the Washington Treaty, the document that established NATO in 1949, a decade before Hawaii became a state.
While Article 5 of the treaty provides for collective self-defense in the event of a military attack on any member state, Article 6 limits the geographic scope of that.
“An armed attack on one or more of the Parties is deemed to include an armed attack on the territory of any of the Parties in Europe or North America,” Article 6 says. It also says any island territories must be in the North Atlantic, north of the Tropic of Cancer."