In the United States v. Hvass case of 1957, the Supreme Court ruled on a matter concerning immigration law and deportation proceedings. The respondent, Mr. Hvass, was an immigrant from Denmark who had been living in the U.S since 1925 but was ordered to be deported due to his membership in the Communist Party between 1939 and 1940. He argued that he did not know about its advocacy for violent overthrow of government when he joined it and left upon learning this fact later on. The issue before the court was whether ignorance or mistake could serve as a defense against deportation under Section 22 of Immigration Act which provides for deportation if someone is found affiliated with any organization advocating overthrowing government by force or violence within ten years after entry into US. The Supreme Court held that ignorance cannot be used as a defense because Congress intended strict liability under this provision irrespective of knowledge or intent regarding such affiliation's nature at time of joining it. Therefore, they affirmed lower courts' decision ordering Mr.Hvass's deportation.

In the dissenting opinion for United States v. Hvass, Justice Brennan disagreed with the majority's interpretation of Section 202(b) of the Immigration and Nationality Act. He argued that this section should not be used to deport an alien who had been convicted of a crime involving moral turpitude before entering the country, if they were unaware at entry that such conviction could lead to deportation. According to him, it was unfair and against legislative intent to apply this provision retroactively without clear indication from Congress. Furthermore, he believed that due process rights under Fifth Amendment would be violated by applying laws retrospectively in a way which creates new legal consequences for past actions. Therefore, he concluded that unless there is explicit statutory command or necessary implication from legislation itself indicating retrospective application intended by lawmakers; courts should presume congressional statutes are meant to operate prospectively only.