The four veteran immigration agents who recently took the stand in federal court had at least two things in common. All were career law enforcement officials. And none could remember ever being asked to make arrests like the ones they carried out earlier this year.

The men said they acted on orders handed down from Secretary of State Marco Rubio’s office in March to detain several noncitizen students, including a doctoral student from Tufts University whose arrest was captured on video and garnered significant attention. Mr. Rubio had abruptly revoked their legal status, which cleared them for detention by immigration agents, citing a rarely used law.

The agents’ testimony — given as part of a trial that concluded in Boston on Monday over the Trump administration’s efforts to deport foreign students who espoused pro-Palestinian views — underscored a major theme of President Trump’s return to the White House. The administration’s tactics have no obvious parallel as Mr. Trump pushes the bounds of executive power and defies legal limits to carry out his agenda.

Challenges to his policies are also, by necessity, treading new ground.

The academic associations that sued over the highly publicized wave of arrests in March have said the government targeted those international students in violation of the First Amendment. In court over the past two weeks, lawyers for the associations argued that the Trump administration stretched Mr. Rubio’s narrow power to revoke visas and green cards in order to stifle the speech of the most vulnerable activists and chill political activity on campuses more broadly.

The government dismissed the notion in its closing arguments on Monday, saying the idea of a coordinated policy targeting noncitizen activists is “the product of the imagination and creative conjuring.”

The Supreme Court has already held that noncitizens in the United States have the same First Amendment rights as citizens in several contexts. Still, constitutional scholars and legal experts have warned that it is precisely because the Trump administration’s actions are so novel that the lawsuit carries some inherent risk, as courts have not previously addressed all the potentially thorny legal questions at hand.