They may, or may not let you in without a U.S. passport. I know someone with dual US & UK citizenship entering with a British passport. They hassled him for a hour but finally said ok.
you have 2 passports, or you can. my brother and sister have both. when they go to canada they use the canadian one, and use the american one coming home. makes the bs questions they ask you at the border that much smoother.
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