Citing Lone Wolf v. Hitchcock as precedent, the Court concluded that Congress retained the power to modify Indian treaties so long as it does so in clear and explicit terms.

Justice Marshall acknowledged that the Court has adopted different standards of review to determine whether Congress intended to abrogate treaty rights.

The Court found adequate evidence in the legislative history that Congress intended to abrogate the treaty rights of Indians.

The Court was unconvinced by the respondent's evidence of an Interior Department memorandum stating that the Eagle Protection Act did not apply to Indian tribes.

Because the legislative history of that Act revealed almost no discussion of its effect on Indian rights, Dion argued that he was immune from prosecution.

Because the issue was not raised on appeal, the Court did not rule on whether Congress' criminalization of hunting eagles violated Dion's religious freedom.