Hans Nielsen was a petitioner in a United States Supreme Court case. He was a Danish citizen who had been living in the United States for over twenty years. He had been convicted of a crime in the state of California and was sentenced to a term of imprisonment. He appealed his conviction to the Supreme Court, arguing that he was not a citizen of the United States and therefore could not be convicted of a crime in the state of California. The Supreme Court ruled in favor of Nielsen, finding that he was not a citizen of the United States and therefore could not be convicted of a crime in the state of California. The Court held that the Fourteenth Amendment did not confer citizenship on foreign-born persons who had resided in the United States for a long period of time. The Court also held that the state of California had no authority to try and convict a foreign-born person who had not been naturalized. The Court's ruling in this case established that foreign-born persons who had resided in the United States for a long period of time were not automatically granted citizenship and could not be tried and convicted of a crime in the state of California. This ruling has been cited in numerous subsequent cases involving the rights of foreign-born persons in the United States.

In the case of Hans Nielsen, Petitioner, Justice Field delivered a dissenting opinion. He argued that the court should not have granted summary judgment in favor of the defendant because there were material facts in dispute and it was for a jury to decide whether or not they had been proven. The majority held that since all parties agreed on certain facts, those could be taken as established without further proof and thus no trial was necessary. However, Justice Field disagreed with this reasoning and maintained that even if all parties agree on certain facts, these still need to be proved at trial before any decision can be made about them. Furthermore he argued that some disputed issues remained which needed to be decided by a jury rather than by summary judgment alone.