He probably does have a cause of action against the US government, too, but they may be entitled to protection from private suits under treaty.
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In any case, Mr. Khadr clearly had a cause of action against the Canadian government, quite independent from any cause of action against the US government.  As the victim of tortious, and almost certainly criminal, acts, he has the right to seek recovery against any of the wrongdoers, or all of them.
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Perhaps Mr. Anglin should read the Supreme Court decision, rather than pandering to the ignorant Conservative base.