The Trump administration wants to halt Harvard from enrolling international students. But how can the federal government dictate which students a private university can and cannot enroll?

The government has enormous power over who comes into the United States, and who doesn’t. For college and universities, the Department of Homeland Security has a vast system just to manage and track the enrollment of the hundreds of thousands of international students studying across the country at any given time.

But a school needs government certification to use this database, known as SEVIS, for the Student and Exchange Visitor Information System. And this vulnerability is what the Trump administration is exploiting against Harvard.

Homeland Security says that effective immediately, it has revoked a certification that allows Harvard to have access to SEVIS. Oddly enough, the students may still have valid visas. But Harvard is no longer able to log them into this all-important database.

The announcement was a significant escalation in the administration’s efforts to pressure Harvard to fall in line with the president’s agenda.

Here’s what we know so far.

How does Harvard use the SEVIS database?

For each international student, Harvard inputs data into SEVIS to show that a student is enrolled full time, and thus meeting the terms of the visa that the student was issued.