IT WAS just minutes after Daniela Vargas appeared at a news conference this month in Mississippi, declaring she would “continue to fight this battle as a ‘dreamer’ to help contribute to this country,” that federal immigration agents pulled her over. Two weeks earlier, agents had arrested her father and brother, undocumented immigrants, at the home they all shared, but let Ms. Vargas be. Now she was the target. “You know who we are and you know why we’re here,” they announced ominously.

Ms. Vargas, who is 22, arrived in the United States in 2002 as the 7-year-old daughter of Argentine immigrants, who then overstayed their visa. She had registered in 2012 and 2014 as a “dreamer,” protected under the Obama administration’s Deferred Action for Childhood Arrivals program, which grants temporary protection from deportation and work permits to migrants brought to the country as youngsters through no fault of their own. Ms. Vargas was in the process of renewing that status, which she allowed to lapse last fall because she didn’t have money for the fee, when she was picked up March 1.

The Trump administration says “the shackles are off” enforcement agents, who in turn have announced that “morale . . . has increased exponentially” in their ranks. The Department of Homeland Security warns that no “classes or categories” of illegal immigrants are exempt from deportation.

In the case of Ms. Vargas, retribution seemed to displace discernment and judgment. Although she has since been released, she still faces possible deportation — a status difficult to square with President Trump’s professed sympathy for dreamers, and with his assertions that deportation efforts would prioritize the “ bad ones.”

Ms. Vargas is a far cry from a “bad one.” A store manager who planned to earn enough to finish her college degree, she hoped to become a math professor, and she had no criminal record. She worked and paid taxes; she was American in every way but by birth. “I would do anything for this country,” she told the Huffington Post.

Agents seemed to grasp that in February when they raided her home and picked up her brother, who has a criminal record, and her father, who doesn’t, but left her alone. That she would be the subject two weeks later of what Immigration and Customs Enforcement called a “targeted immigration enforcement action” — directly after leaving a news conference called by immigration advocates in which she had expressed concern about her family — looks like official vindictiveness.

By prioritizing illegal immigrants who were serious and violent felons, as well as recent border crossers, the Obama administration frustrated many ICE and Border Patrol agents, who felt constrained from making collateral arrests. Now, the danger is that the pendulum is swinging in the direction of arbitrary and senseless enforcement. Agents recently have detained migrants with no criminal records, including outside a church shelter in Alexandria. In sweeps last month, agents detained nearly 200 migrants with no criminal history, in addition to hundreds more with criminal records. The best means of avoiding such excesses is for Congress to exercise effective oversight, and soon.