US rule could keep Iroquois from lacrosse tourney

As I understand it The 6 nations Iroquis are a sovreign nation.

No they are not, American Indian "nations" are not sovereign entities. They are self governing autonomous regions of the United States - meaning they have internal self government over domestic matters but things such as defense, currency, foreign affairs, and citizenship (including PASSPORTS) are handled by the US federal government hence they must use US passports.

Their status is somewhat similar to a British crown dependency (the Isle of Man, and the Channel Islands) in that even though they are technically not part of the UK they are considered to be British subjects and use British passports, rely on the British military for defense, and depend on the UK for "good governance" and foreign affairs matters.