A more thorough explanation:

United States v. Wong Kim Ark is a landmark case in which the Supreme Court held that citizenship as prescribed in the Fourteenth Amendment extends to U.S.-born children of foreign subjects or citizens who, at the time of the child’s birth, are permanent residents and are carrying on business in the United States. Such children acquire U.S. citizenship at birth, but this does not apply if the parents are in the United States in any diplomatic or official capacity.

For example, Wong Kim Ark was born in San Francisco in 1873 to laborers of Chinese descent. His parents were Chinese subjects but maintained a permanent domicile in San Francisco. After arriving back from a temporary visit to China in 1895, Wong Kim Ark was detained at the port of San Francisco and refused permission to land. This was due to the Chinese Exclusion Act, which came into force in 1882, in general forbidding Chinese persons from entering the United States and Chinese residents from naturalizing as citizens. Wong Kim Ark contended that he was a U.S. citizen as prescribed by the Fourteenth Amendment due to his birth in the United States, and therefore the Chinese Exclusion Act did not apply to him.

The Court first noted that there is no statutory definition of a citizen, except the inclusionary clause in the Fourteenth Amendment stating that “all persons born or naturalized in the United States, and subject to the jurisdiction thereof” are citizens. The Court therefore relied on common law to interpret this and other clauses concerning citizenship. The main principle that the Court chose to draw was from Calvin’s Case, a 17th century English common law case that held a person born within the territory of a King owes him allegiance, and is therefore the King’s subject. The Court then referenced a series of commentaries and cases in both English and U.S. common law that showed subsequent decisions since Calvin’s Case have been consistent with this principle.

The Court was also interested in the circumstances surrounding the adoption of the Fourteenth Amendment. It pointed to the Civil Rights Act of 1866, which was passed by the same Congress that adopted the Fourteenth Amendment, stipulating citizenship for “all persons born in the United States, and not subject to any foreign power, excluding Indians not taxed.” From this the Court reasoned that “the opening sentence of the Fourteenth Amendment is throughout affirmative and declaratory, intended to allay doubts and to settle controversies which had arisen, and not to impose any new restrictions upon citizenship.”

In conclusion, United States v. Wong Kim Ark established that children born in the United States to foreign parents who are permanent residents and carrying on business in the United States are U.S. citizens at birth, as prescribed by the Fourteenth Amendment.