Green v. United States was a Supreme Court case that addressed the issue of whether a defendant could be convicted of a crime if the indictment was not found by a grand jury. The case was brought by William Green, who was convicted of a crime without a grand jury indictment. Green argued that the Fifth Amendment of the United States Constitution required that a grand jury indictment be found before a defendant could be convicted of a crime. The Supreme Court ruled in favor of Green, holding that the Fifth Amendment did indeed require a grand jury indictment before a defendant could be convicted of a crime. The Court reasoned that the Fifth Amendment was intended to protect citizens from arbitrary and oppressive government action, and that a grand jury indictment was necessary to ensure that the government was not acting arbitrarily. The Court also noted that the grand jury was an important check on the power of the government, and that it was necessary to ensure that the government was not acting in an oppressive manner. In conclusion, the Supreme Court held that the Fifth Amendment required a grand jury indictment before a defendant could be convicted of a crime. This ruling was an important step in protecting citizens from arbitrary and oppressive government action.

In the case of Green v. United States, Justice Field delivered a dissenting opinion in which he argued that Congress had exceeded its constitutional authority by passing legislation to punish an individual for acts committed prior to the passage of such legislation. He reasoned that this violated the Ex Post Facto Clause of Article I, Section 9, Clause 3 of the Constitution and was therefore unconstitutional. Furthermore, he noted that even if Congress had not exceeded its powers in enacting such laws it would still be unjust and oppressive as it would allow individuals to be punished retroactively for actions they could not have known were criminal at the time they were committed. In conclusion, Justice Field argued against punishing individuals under ex post facto laws because doing so violates fundamental principles of justice and fairness enshrined within our nation's constitution.