WASHINGTON — In his first year in office, Attorney General Eric H. Holder Jr. put new limits on when the government could dismiss lawsuits in the name of protecting national security. Now, in what he has said is likely his final year, Mr. Holder has claimed broad authority to do just that in a case unlike any other.

The Justice Department intervened late Friday in a defamation lawsuit against United Against Nuclear Iran, a prominent advocacy group that pushes for tough sanctions against Tehran. The government said the case should be dropped because forcing the group to open its files would jeopardize national security.

The group is not affiliated with the government, and lists no government contracts on its tax forms. The government has cited no precedent for using the so-called state-secrets privilege to quash a private lawsuit that does not focus on government activity.

The lawsuit, by a Greek shipping magnate, accuses United Against Nuclear Iran of falsely accusing him of doing business with Iran. The businessman, Victor Restis, subpoenaed the group for its donor list and all information it had collected about him. That was when the Justice Department stepped in.