In United States v. Morillo, the Supreme Court of the United States held that a person who is not an American citizen cannot be convicted of treason against the United States. The case involved Pedro Morillo, a Spanish national living in Florida during the Civil War. He was accused of aiding and abetting Confederate forces by providing them with supplies and information about Union troops' movements. At trial, he argued that since he was not an American citizen, he could not commit treason against the United States as defined by Article III Section 3 of the Constitution which states “Treason against the United States shall consist only in levying war against them or adhering to their enemies giving them aid and comfort” Since citizenship is required for one to levy war or adhere to enemies under this definition, it follows that non-citizens are unable to commit such acts according to U.S law. The court agreed with his argument and overturned his conviction on appeal ruling that non-citizens cannot be charged with treason because they lack standing under Article III Section 3's definition of what constitutes treasonous behavior

In United States v. Morillo, the Supreme Court was tasked with determining whether a person born in Puerto Rico while it was under Spanish rule could be considered an American citizen and thus subject to military conscription. The majority opinion held that such persons were not citizens of the United States and therefore could not be conscripted into service. In his dissenting opinion, Justice Field argued that Congress had intended for all inhabitants of territories acquired by the U.S., including those born before acquisition, to become citizens upon annexation or cession of said territory; as such he believed they should have been allowed to serve in the military if called upon to do so. He further noted that this interpretation would help ensure loyalty among these new citizens towards their adopted country since they would now have a vested interest in its success and defense against foreign powers due to their own involvement in defending it from attack or invasion through military service when needed.