In Robert's opinion in Trump v Barr, he mentions that the idea that a president is subject to subpoena dates back to the federal government's treason trial against Aaron Burr.
Burr argued that Jefferson had papers that would exonerate him, and that he had a 6th amendment right to that material, the court agreed. In addition, the precise language in the Constitution was used to acquit Burr.
- Here is a description of the trial.
For detail on the Burr Conspiracy, which involved the territory which would later become Texas, click below.
Wikipedia: Burr Conspiracy.
Burr's trial brought into question the ideas of executive privilege, state secrets privilege, and the independence of the executive. Burr's lawyers, including John Wickham, asked Chief Justice John Marshall to subpoena Jefferson, claiming that they needed documents from Jefferson to present their case accurately. Jefferson proclaimed that as President, he was "...Reserving the necessary right of the President of the U S to decide, independently of all other authority, what papers, coming to him as President, the public interests permit to be communicated, & to whom..." He insisted that all relevant papers had been made available, and that he was not subject to this writ because he held executive privilege. He also argued that he should not be subject to the commands of the judiciary, because the constitution guaranteed the executive branch's independence from the judicial branch. Marshall decided that the subpoena could be issued despite Jefferson's position of presidency. Though Marshall vowed to consider Jefferson's office and avoid "vexatious and unnecessary subpoenas," his ruling was significant because it suggested that, like all citizens, the President was subject to the law.
Marshall had to consider the definition of treason and whether intent was sufficient for conviction, rather than action. Marshall ruled that because Burr had not committed an act of war, he could not be found guilty (see Ex parte Bollman); the First Amendment guaranteed Burr the right to voice opposition to the government. To merely suggest war or to engage in a conspiracy was not enough. To be convicted of treason, Marshall ruled, an overt act of participation must be proven with evidence. Intention to divide the union was not an overt act: "There must be an actual assembling of men for the treasonable purpose, to constitute a levying of war." Marshall further supported his decision by indicating that the Constitution stated that two witnesses must see the same overt act against the country. Marshall narrowly construed the definition of treason provided in Article III of the Constitution; he noted that the prosecution had failed to prove that Burr had committed an "overt act," as the Constitution required. As a result, the jury acquitted the defendant
