What did Donald Trump do today?

He completely froze one of the largest American industries.

International students represent an enormous source of income for the United States, adding $44 billion to the economy and supporting almost 400,000 jobs, by one conservative estimate. Foreign students are a source of prestige, political power, and global influence for the United States. They also burnish American higher education's standing as the gold standard not only for overall quality, but for its contributions to science, technology, and medical research. By admitting the very best students from foreign countries, the United States gains not only the benefit of their talents, but also the chance to bring them within the fold. 

In other words, for the better part of a century, the United States have been the greener pastures that have lured the world's most promising students and researchers. "Brain drain" has worked exclusively to the United States' benefit for all that time—at least, until now.

The result of all of this is an enormously profitable and symbiotic relationship between American colleges and universities and the roughly 5% of their student bodies that are from countries other than the United States. The United States receives a financial benefit bigger than the profits from almost any other sector of the domestic economy: for example, it brings in twice as much foreign money than the entire iron and steel industry. As one recent editorial put it, foreign students are "the greatest free lunch in history" for the United States.

But not everyone is allowed to study in the United States. Students must rigorously demonstrate that they are able to financially support themselves before coming to the United States. Countries hostile to the United States cannot send students to American universities. Colleges and universities must vouch for the presence and academic progress of all foreign students they admit. And all international students must be interviewed by an American consular official.

As of today, Trump has halted all consular interviews, effective immediately and indefinitely.

The stated rationale for the freeze is to increase scrutiny of prospective students' social media accounts, to scan for political ideologies that contradict the Trump administration's views—which is to say, defining any such contradictory views as "antisemitism" or "terrorism," regardless of their content. Normally, international students choose American universities precisely because they won't be subjected to restrictions on their political speech. But in a transparent attempt to change that perception, Trump has had international students with valid visas imprisoned for views they expressed long before he even took office, and which are protected by the First Amendment.

A spokesperson refused to give further details, or to guarantee that the freeze would be lifted before it effectively destroyed international student admission for the fall semester.

Why does this matter?

• It's only "terrorism" to contradict the leader in a dictatorship.
  

• International students, even more so than immigrants in general, are not a threat to the United States.
  

• There are no good reasons to want to undermine the greatest system of higher education in the world—but there are a few evil ones.