What is an appropriate punishment for a crime? The plight of thousands of Vietnamese refugees convicted of crimes in the United States and now threatened with detention or deportation demands an answer to this question.

For most of the four decades since the Vietnam War ended, refugees fleeing the Communist government in Vietnam have been treated differently from those refugees from most other countries. If they committed felonies, they would not be deported.

This exception tacitly acknowledged the weight of history: These refugees came here because American forces fought a devastating war in their home country, the Republic of Vietnam, or what Americans called South Vietnam. The exception recognized that America owed a measure of compassion to those who lost their homeland in the wake of the Communist invasion of the South, and the United States military intervention to resist it.

All that began to change under Donald Trump. Mr. Trump has set his sights on deporting more than 8,000 permanent residents from Vietnam, most of whom had fled the war and had been protected from removal despite their crimes. Mr. Trump’s efforts drew sharp criticism not only from Vietnamese-Americans , but also his own ambassador to Vietnam, Ted Osius, who eventually resigned from the State Department in protest.